> the web has very good backwards compatibility guarantees

Kinda... Google and folks have been cracking down on security pretty hard, to the point where certain things would probably stop working if you weren't maintaining the security of the endpoint or something correctly. There are APIs (more and more everyday it seems...) that only work with "secure contexts" like HTTPS, and they're working actively on tightening HTTPS requirements (like shortening certificate lifetimes, valid ciphers etc). Sure, this helps improve security, but not without breaking compatibility.