You can make hardware completely Backwards compatible and not ask a single developer. When Nintendo made the Wii it was completely backward compatible with the Gamecube. You can play any Gamecube game on the Wii. Sony also did this with the PS2 as it was able to play PS1 games no problem. Developers can't legally stop the Xbox 4 and PS5 from being backward compatible like the Wii. However, Developers can pressure Sony and Microsoft to not do it by support means. Which means that they will treat the system with no backward compatibility with better support.
Honestly making a system backward compatible is a lot of work. You have to basically make two consoles in one. Which is why a lot of systems don't do it. Microsoft only has to ask developers to add a game to BC because digital rights are different from physical rights.