What about Xbox One and Xbox One S though? It proves it can be done right.
Different situation imo.
Xbox One S is the same architecture of Xbox One. There is just a really small upclock in the GPU. Chances of ruining compatibility are really low.
The PS4 Pro, if you read the Mark Cerny article, has many internal differences compared to the standard PS4 and an interesting point he makes is that they are using a mirrored second GPU they turn off to keep compatibility in old titles.
Chances of fucking up compatibility with old titles are much higher compared to Xbox One S and having all the PS4 games working flawlessly is vital for a system that IS a PS4.
Do you expect Microsoft to let Xbox One games running on Scorpio outside of a compatibility mode for example?