I wouldnt go that far in saying x86 guarantees BC. When you read interviews with Cerny about how he developed the PS4 pro hardware, he was very cautious about how to approach it so that the machine can still run in a base PS4 mode with the same hardware. Even changing the GPU slightly would cause issues os if the CPU ran at a different clock speed for current games on the system. He also said it be difficult for MS to go without generations because of changing gpu architectures(however we are talking about a company who's emulation abilities and legendary).
This might be due to the fact that Xbox and PS4 dev kits allow developers to target the hardware directly. So if sony goes with a different CPU and GPU architecture, I wonder how they will handle BC.
I could be wrong though, but based on the interviews Cerny did, didnt give me much hope for BC on PS5. If I am wrong please someone reassure me as I have games on my PS4 that I own digitally that id like to keep on PS5.
I think it's a difference in how each company operates. Sony restricts software from utilizing Pro hardware unless there's a patch, with Cerny, as you stated above, having concerns about compatibility. Xbox One S is more powerful than Xbox One, and MS lets any game have access to that power, without a patch. Spencer has said in the past that Scorpio will work in a similar manner, providing benefits even to games which are not formerly patched for it, including Xbox 360 BC games.
While there may be some challenges, I think it's safe to expect MS engineers are motivated and capable to ensure BC and forward compatibility for their platforms.
Seeing as you're talking about PS5 here, it's simpler to then also talk about the Scorpio successor.
Will games you buy on the Scorpio 2 in 2020 still work on XB1? You're saying they will, but I have my doubts. Forward compatibility only goes so far.
I think it's unreasonable to expect games to continue working on XB1 in perpetuity. I think, like PC/mobile, there will be a point where *mandatory* support for Xbox One is no longer required, perhaps MS internally has a date pegged, but for not we can only speculate how that will work out. My guess would be when the Scorpio successor is released, mandatory support for Xbox One is no longer required - MS would get a shifting 2 console paradigm going forward, High (Scorpio) and Low (Xbox One).