Since the PS4/PS4 Pro/PS5 all use AMD tech for CPUs/GPUs it will be super easy to support older PS4 games on PS5.
The comparison between the Xbox One and 360 you're making is totally different. While they both use AMD for their GPUs they use totally different CPUs. The Xbox 360 has a three core
3.2 GHz IBM PowerPC CPU. It's not even x86 compatible. So they have to use an emulator to get the games working. Totally different thing.
No it won't be, the CPU and GPU are going to be brand new and LIKE PCs when you release new Hard ware you spend months to years to FIX broken components and some Software is never upgraded and Brakes perminantly on that GPU and OS.. Not only are they adding NEW hardware there will be an new Operating system build thats going to have to run it...
Yes they are going to have to spend Millions doing complaince checks but Even with software or hardware, there Past has Shown a high failure rate and Critical Bugs. can kill a game engine and kill the game...
Driver compliance is an issue even on windows or Linux or mac when some one releases a NEW GPU with out testing every single game, the OLD games can fail... this then requires patches for the new GPU ethier Game or in firmware...
The I9 intell chipsets is still having firmware patches in OSes to fix critical Issues that crash games and applications that no other chipset has because it was NEW...
The AMD APU powering the PS5 will be "custom" that includes custom critical bugs running PS4 software, that are not present on other APUs and thats the issue, trying to force a new chipset that the games never ran on is going to cost SIE lots of money And the user will end up paying for BC even if they don't use it...
Same with VR forcing compliance on new hardware will cost SIE R&D money the User will have to pay that back and if the Break out box is built in this time as was rumoured every user will be paying for that break out box being included in PS5 and SIE will want that R&D money back...