Deciding whether and when to upgrade macOS is one of the more difficult choices we face. If your Mac isn’t capable of running the current release of macOS, or you’re dependent on key hardware or software which is incompatible, then the decision is made for you. For most of us, though, there’s nothing really holding us back except our own decision, which should be informed by facts rather than false hopes or assumptions. Let me question some common perceptions.

Apple supports macOS for three years

For several years, I’ve been searching for the document in which Apple might have stated this common assumption, and so far I have been unable to find it. Just over a year ago, I examined this in detail, concluding that “over a period of eight years, Apple has followed what most believe to be its policy on macOS support: major versions enjoy full support for the year that they are the current release, then receive approximately two years of security updates.”

To bring this up to date I have analysed the Monterey cycle, between its release in October 2021 and that of Ventura last month. Over that period of exactly a year, Apple released non-security updates for Monterey only, with a single exception: on 9 June, one severe bug affecting the opening of mail attachments was fixed in Big Sur 11.6.7, an extremely unusual event.

All support for Catalina ceased on 20 July, with Security Update 2022-005, and Monterey’s last non-security bug fixes were released that same day in 12.6.

By the time that the new version of macOS is released in the autumn/fall, the previous version typically hasn’t had any fixes to bugs that don’t affect security vulnerabilities for at least two months, making its period of full support less than ten months.

Apple’s security updates are sufficient

When the current version of macOS loses its general support, and starts its two years of security-only fixes, there are still many bugs left in it, which are only likely to be fixed in the new version.

Those residual bugs are often severe: El Capitan continued to cause many Macs to grind to a halt through its entire cycle, the cause not being addressed until Sierra. Sierra in turn was abandoned with a serious bug in its backup scheduling system that caused automatic backups to fail completely after a few days, and that was only fixed in High Sierra. More recently, Monterey initially suffered from three serious memory leaks, of which two were fixed early, but the third has only just been addressed in Ventura.

What I’ve not seen considered by anyone are updates to APFS. For the year that it’s supported, each version of macOS gets a new version of APFS with each minor update, and sometimes in intermediate security patches too. Apple hardly ever mentions bugs or fixes to APFS, but some known problems have come and gone, only in the latest version of macOS, of course.

We have no idea what bugs in old versions of APFS are known to Apple, nor of their consequences. Just at the moment, though, I’m concerned that I’ve heard of a few users whose APFS Encrypted external disks have suddenly become unusable; as most have been storing Time Machine backups, those users have lost all their backups. APFS is but one example of a critical sub-system that loses all support once Apple releases a new version of macOS.

Apple hasn’t provided detailed lists of more significant bugs fixed in updates for many years now, but does provide itemised release notes for fixes addressing identified security vulnerabilities.

Many of us had suspected that, during those two security-only maintenance years, older macOS didn’t get all the fixes it could have. I wrote that those security updates “address many of the more significant vulnerabilities which are found in it”. That was examined in detail a year ago by Joshua Long, who concluded on Intego’s Mac Security Blog that “unless you’re running the very latest major version of macOS (now macOS Monterey), Apple’s updates provide only selective fixes along with a false sense of security.”

Most recently, Apple has confirmed this: “Because of dependency on architecture and system changes to any current version of macOS (for example, macOS 13), not all known security issues are addressed in previous versions (for example, macOS 12).” (Oddly, that latest revision may not appear yet on all localised versions of that guide, including the UK version.)

This is visible when you simply count the number of vulnerabilities fixed according to Apple’s release notes.

Over the 2021-22 cycle, the current version of macOS, Monterey, received a total of 342 security updates, Big Sur only 202, and Catalina just 146. The graph above shows how those three versions diverged over that time. Monterey started with an advantage, in arriving with all the fixes accomplished during its development. Its overall gradient is greater than either Big Sur or Catalina, and towards the end of the cycle their rate of fixes slowed, in Catalina to flatline after July, when it was abandoned altogether.

This will soon be amplified by Ventura’s new Rapid Security Response, which will deliver smaller security patches several weeks before they’re likely to be incorporated into full macOS updates. Those Macs upgraded to Ventura will thus be patched long before those still running Monterey or Big Sur even stand a chance of those being incorporated into their next security update.

New versions of macOS are full of bugs

macOS is vast, and I don’t think anyone can hazard a guess at how many identified bugs might exist in any version. What is more feasible is discovering whether there are any serious bugs, such as those causing kernel panics, major memory leaks, etc., that many users are likely to encounter. Although the risk of those is likely to be highest in the first version of any release cycle, they can be introduced in any update after that. One of the most serious and frequently encountered in recent years appeared in Catalina 10.15.6, for instance.

One factor that makes recent memory an unreliable indicator is the series of changes to macOS to prepare for Apple silicon Macs. Those started in earnest with High Sierra and the introduction of APFS, continued in Catalina with the loss of 32-bit support and the new Boot Volume Group, followed by Big Sur’s signed and sealed system volume (SSV).

In contrast, upgrades from Big Sur to Monterey, and from Monterey to Ventura, have generally been far less traumatic. Indeed, with the new mechanism for updating and the demonstrable integrity of the SSV, those versions enjoy more reliable updates and robust System volumes. At least malfunction is now likely to be a bug rather than the result of damage to the system.

It’s safer to delay upgrading

Much as I respect Andrew Cunningham in Ars Technica, I question his conclusion that “most people running an up-to-date Big Sur or Monterey installation with an up-to-date Safari browser should be safe from most high-priority threats, especially if you also keep the other apps on your Mac updated.” Even if security threats were the only concern, this begs the question as to how to tell whether you are among those “most people”, and the only threats you’ll ever encounter are among the “most high-priority threats” that you should be safe from.

It’s a similar argument to that concerning vaccination of younger people against Covid or other generally non-fatal conditions. Most people under the age of 70 who are in reasonably good health should be safe from serious or prolonged illness or death. But the only way to tell whether you are most people is in retrospect.

Mark Josh Long’s words: “macOS Mojave is — and presumably always will be — vulnerable to the “FORCEDENTRY” bug that has been actively exploited by the Pegasus spyware.” What other actively exploited bugs are Big Sur and Monterey now vulnerable to? What bugs remain in their old versions of APFS, Time Machine, or anything else in the system that could result in data loss?

Making these decisions is always a gamble. Although I well understand you might feel more reassured by waiting a couple of weeks to ensure there are no showstoppers that might make upgrading unwise, delaying beyond the first update brings ever-increasing risk. That’s no longer a risk I’m prepared to take: I put my production Mac where my mouth is, and upgraded it to Ventura on the day of its release, followed rapidly by my other Macs.