Because who doesn't want better battery life for their desktop machine? Or quieter fans or more memory.
Desktops have plenty of RAM, enough to not worry about whatever insignificant benefits you're talking about that may arise from ditching backwards compatibility. Which by the way, has never been an undesirable feature. You're just trying to sell it as undesirable by claiming users are in great debt to those who decide to take away the option to run older software.
Apple has stated they will delete iOS apps from app store if they haven't been updated in awhile even if they technically still function. This will lead to a farcical situation of updates for the sake of updates, which is no doubt happening now anyway to get attention. "Remember me? I'm the app you haven't touched in a few months, here's an update!"
You're a subscription apologist and you dislike backwards compatibility. Your position is unconvincing in this discussion, but it sounds like you're happy! All the best.
Photoshop actually is another example of things working as they should. Adobe has ported Photoshop from 68K, to PPC, to OS X, to x86 32 bit, to x86 64 bit, to ARM.
As far as subscriptions, I don’t use Photoshop. But back in the day, one seat of MS Office use to be $600. Now I can get a 5 user license of Office where each user can use Office on any combination of Macs, Windows PCs, iPhones, iPads and the web plus 5TB of cloud storage for $99 a year.
The vast majority of people are using laptops. But even desktop ARM Macs are seeing the same benefits from being able to ditch backwards compatibility.