Decentralising NIPs might break the indexing completely. We might have seventeen NIP-41s and no way to do a key rotation reliably.
That being said even if the NIPs repository is centralised it has no way to enforce bad decisions in a tyrannical way. Client devs are "free farmers" and can do whatever they want but it is in their best interest to follow the interoperability train most of the time. The users keep client devs in check.
A centralised NIPs repository is not a bad thing.