Next-gen (current-gen) consoles are so hard to come by lately. It'll be over a year minimum that supply will catch up to demand (with chip shortages etc...).
That you can offer this to console owners who just can't upgrade is a great fan service tbh. Games like The Medium that are next-gen only (and future titles as well) could be played with their current consoles on the big screen (as opposed to a tiny screen on their phone).
MS could focus on making games exclusively for Series X (without being shackled by last-gen) and yet still have those games playable on last-gen consoles.