You know what pains me most? I, too, work on a 2017 Retina iMac 27" and I just love the screen.
I get that (I have a 2017 iMac gathering dust & bought new, non-Apple displays for my Studio) but on the other hand that's why, going forward, I much prefer the Mac Mini/Studio concept - we can upgrade our displays and computers independently. I
wouldn't have bought an all-in-one in 2017 if Apple had offered something like the Studio at the time (the trashcan was already abandonware at that stage). I'm not a huge fan of the Studio Display for various reasons (q.v.
ad nauseum elsewhere) but its certainly not rubbish and - unlike the iMac screen - should serve several generations of connected Mac. If you
need a new desktop Mac now, I'd suggest looking for cheaper, alternate displays which you wouldn't mind replacing - or delegating to second screen if something more compelling comes out during the lifetime of your current machine.
It does open up new possibilities - I've gone for a
pair of 3:2 4k+ 28" Mateviews which may not quite match the iMac/Studio Display in raw quality, but which I'm finding far more suited to my needs than a single 5k.
Why does that prove next year’s macOS will be silicon only? Breve the lineup for hardware has all transitioned?
It doesn't
prove anything - Apple will drop Intel support from future OS versions just as soon as they can get away with it publicity wise. Apple never promised anybody support for future, unannounced versions of MacOS! They're obliged, in some jurisdictions, to provide spares and support for sometime between 5-7 years, and now that they have stopped selling the last Intel Mac has that 5-7 year old countdown started - but
at most that requires them to have
a minimally-supported version of MacOS that will run on those systems.
It was never likely - and would have been rather unpopular - but they
could have made Sonoma Apple Silicon only, and extended minimal support for Ventura for a couple of years.
Realistically, all we have to go on is that Mac OS 10.6 (Snow Leopard) was the first to drop PPC support in 2009 - 3 years after the Intel transition completed in 2006.
You can also look at the fact that Sonoma is dropping support for models that were last sold in 2019.
On the other hand, since Ventura, many of the new features have been Apple Silicon only.
If I had to guess, I'd say that today's Apple will be in a hurry to drop Intel support and next year's Mac OS will be the last Intel-supporting one - but that's a pure guess.