Not sure why I’m posting this.
@cateye has posted a much superior analysis and I only dilute things by posting an anecdote rather than a hard and fast rule set. But hey, I’m human (mostly)… I’ll cite “horses for courses”
As my M1Max is more than I require for regular use (M1Pro would likely have been sufficient but I like to future-proof) and will remain so until I get off my duff and actually chase an old dream (never? Later…), I anticipate using it for a few years still with no concern whatsoever for dropped macOS support. When I replace it, it will be because a new Mac offers Thunderbolt 5 (for the video bandwidth needs of the next generation of top end monitors) as well as my return to the desktop fold (Studio here I come). Perhaps I’ll have started requiring more of it than it can provide by then as well (doubtful), however it will be fully covered by many remaining years of macOS updates.
My previous machine, a ”fully loaded” 2014 5K iMac, was still in limited use last year (it was far from a powerful computer even when new, CPU aside). Started feeling its age, though and support had been dropped for it in macOS, and then I do not keep them. I don’t tinker meaningfully with old hardware and have a personal requirement that the device is at least still getting security updates if I’m going to use it.
I expect the M1Pro/Max generation to last a good long while: if RAM becomes the cutoff point, they’re at 16+GB and the M3 still starts at 8GB. If power does, the base M1Pro handily outperforms an M2 (excepting single core) and likely an M3 in CPU, decently in GPU, matches them in video encoders/decoders (excepting AV1 on the M3), and only comes up short when ray tracing and mesh shaders enter the scene (M3, and whether Apple’s implementation of them is any good has not been tested). Anybody want a gentleman’s bet that macOS won’t drop M1 support until 2028 at the earliest and that there’s a greater than 50% chance the M1 Pro/Max/Ultra chips retain support for another release or two thereafter?
Intel Macs, on the other hand, are dying a rapid death in macOS and that is not likely to decelerate much. Although I dislike that pace, I expect presently supported Intel Macs in macOS to remain supported in the next version (15) in autumn 2024 (Intel Macs with T2 chips can still be plenty fast and secure) and then either completely deprecated or on deathwatch the macOS thereafter (16), autumn 2025 (the absence of ”neural” cores seals their fate).