True stability is impossible for almost any language out there, especially one with a nontrivial type system.

An example of a thing considered "trivial to upgrade" over is when the stdlib adds a new method to a type. Libraries may already have implemented a method of the same name on that type via a trait. In this case, folks upgrading would have to explicitly specify which method they're talking about.

It's these kinds of things that are not counted as breaking changes, because the alternative is freezing the stdlib forever. Other languages have the similar problems.

New releases are tested on the entire ecosystem to ensure that this rarely happens, if ever.