Sometimes, Apple’s behaviour looks baffling. One of its biggest-selling and flagship computers, the iMac, was left without any hardware upgrade for over twenty months. Then, when it announced new models last week, it slipped them out in a press release just a few days after an Event to rejig its range of iPads.

Not only that, but after more than a year of telling us how its T2 chip is central to the future of Macs, and shipping them in its basic Mac mini costing £/$/€ 799, not one of the new iMacs even offers it as an option. To get an iMac with a T2 chip, you still have to step up to an iMac Pro starting at nearly £/$/€ 5000.

We should ask why.

Explanations start to emerge when you look at just what these new iMacs are: the previous models with an Intel Coffee Lake (8th and 9th generation) CPU and chipset, faster memory, and updated graphics cards reaching up to the Radeon Pro Vega 48. They still have only two Thunderbolt 3 ports, whilst the MacBook Pro 2018 and the Mac mini 2018 (even that basic £/$/€ 799 version) sport four.

Changes required to the previous logic board design, based on a Kaby Lake chipset, should have been small compared to those required to integrate the T2 chip, for example.

Apple is signalling to us that, if we’ve been waiting for updated iMacs to ship, these are safe to buy, but if you’re waiting for a proper new model, comparable with last year’s updates to the MacBook Pro, MacBook Air or Mac mini, then you’re going to have to wait longer. Which is strange given the iMac’s previous place in Apple’s product range, since it first caught on back in 1998 – yes, over twenty years ago.

So why is Apple not ready to launch the really new iMac yet?

Cast your mind back to 2005-6, when Apple switched from PowerPC to Intel processors. Which was its first desktop product to make that leap? The iMac. Apple had seeded developers with a special edition based on the Power Mac G5 with its cheesegrater case soon after announcing the switch at WWDC 2005, and within seven months was shipping iMacs and MacBook Pros with Intel inside.

If Apple intends releasing its first Macs incorporating its own ARM-based processors in, say, 2020, wouldn’t it be probable that those initial models would again be the iMac and MacBook Pro? Following the previous, and highly successful, timetable, that means announcement at WWDC this June, developer systems soon afterwards, software support in macOS 10.15, and production models early in 2020.

So – if the rumours about switching to its own ARM-based processors are true – the latest new iMac is probably the last to be built around Intel processors, and a stopgap before the big news of the iMac 2020. This also fits neatly with macOS 10.15 requiring 64-bit software, as Apple won’t want to have to continue 32-bit support when it will, for some time at least, be supporting two processor architectures again.

If Apple is running on a longer timetable for the introduction of ARM-powered Macs, then these new iMacs would have to serve as stopgaps for more than a year, as it looks unlikely that Apple intends doing more with them until that complete hardware overhaul.

So, should you risk buying such an interim iMac?

If your existing Mac is over three years old, and you want to replace it within the next year or so, then they should be a safe purchase. If your Mac can reasonably soldier on for another three months, wait until you hear what Apple announces at WWDC in early June. If there’s no mention of new processors, or a major iMac redesign, that’s the best time to decide whether to buy one.

They’re a good incremental update if you can’t justify going up to an iMac Pro, or any new Mac Pro which might be announced in the summer, but I can’t help feeling that much greater change isn’t that far away.