In the end it’s only temporary. Xbox does have the upper hand long term. Right now it’s more in their interest to take the BC gaming preservation route because Sony has a bad service by comparison. BC is usually a requested feature in the first year until it dies off in the 2nd. We know Xbox is more powerful, MS will show it in due time.
However PS5 will have close to 100% backward compatibility too according to Cerny, it's not like it's a xbox exclusive feature. And I'm not sure early adopters are much interested in backwards compatibility anyway. They're not buying a new console in the first few years to play old games at higher resolutions or frame rates. People that are willing to spend for those things are on pc already. At the end of the day, the console running the most visually impressive and generally better games will win, as always, regardless of native rendering resolution, frame rate and BC.