You'd have be cuckoo to spend anymore money on Xbox store on console without gurantees it will be working via BC on this new "console".
There's PlayAnywhere (which seems to be on the rise).
There's the digital legacy of the catalog in the ecosystem (PC, Console, Xcloud).
And clearly, there's going to be a plan for preserving games, and a plan for preserving game content (BC) has been announced.
That is, assimilating the catalog to PC (where all those games are also) should provide more security that you'll be able to continue accessing that catalog on future hardware and other devices with more security and even in better conditions like resolution, FPS, better graphics (without having to pay for patches

).
I don't know, I don't think there's anything to fear more than with any change or arrival of a new generation (BC didn't even exist on several).
Perhaps the fear would lie with those with physical games and playing the content on that disc?
1- That doesn't happen anymore because when you put in the disc, you only activate a download license for the patched digital copy.
2- In the end, 99% of users buy games to play them on the current hardware where these always be, and 90%+ only buy digital games.
But hey! We know, the typical opportunity to invent a reason to insult users of other platforms

.
Considering how modern PC games run poorly and have a lot of stuttering issues + crappy DX12, this is going to be a lot of fun.
Who knows? What if it's an opportunity for PC games to appreciate optimization improvements by having a mirror platform that will surely receive help from MS to create suitable and optimized versions (DevKit, tools, Pix, XDK, etc.)?
With the rise of AI (all future hardware will be AI-based), many of these problems will likely be solved. I think the sensible thing to do would be to wait and see how it turns out and how they can execute it before making any judgments.
?? I don't know what FSR4 could do to fix or solve the problems you mentioned.