I think “the fifth revision of that URL routing library that everyone uses” is a much less common case than “crate tried to explore a problem space, five years later a new crate thinks it can improve upon the solution”, which is what Rust’s conservatism really helps prevent. When you bake a particular crate into std, competitor crates now have a lot of inertia to overcome; when they're all third-party, the decision is not “add a crate?” but “replace a crate?” which is more palatable.

Letting an API evolve in a third-party crate also provides more accurate data on its utility; you get a lot of eyes on the problem space and can try different (potentially breaking) solutions before landing on consensus. Feedback during a Rust RFC is solicited from a much smaller group of people with less real-world usage.