I think the thing, to me, is that semver is very useful where these things are purely mechanical. This (at least usually) applies to libraries. Semver enables things like lockfiles as used by ruby bundler and later cargo and various other things. But not all systems are upgraded in mechanical ways, and not all 'breaks' are purely mechanical.
This is why you can't really use semver (usefully) for everything. What's a "breaking change" in a word processor?
Languages aren't used like word processors, but they also aren't exactly used like libraries either. People get stuck on language versions for different reasons than why they get stuck on library versions.
At any rate I think in practice I think languages that are trying to hew to semver concepts like this just wind up with a 'fake major version'. Since Rust, for eg., might never go to 2.x the 1. in front of 54 is really just academic. That's the "real" major version as far as anyone needs to know.
A lot of the arguments in this thread seem to be kinda tautological. There's no law that says they have to use semver, nor is there a law that says semver can't be imperfect. "Semver is semver because semver says so" is not a compelling argument.
