Backwards-compatibility is held extremely dear to the C community, with those few breaking changes being simple (usually) to work around. If you want to break compatibility, you have a better chance calling it a new language.
Thats true, but new modes can be added with switches similar to how gcc has -std=c89, -std=c99 and whatnot. Newer modes could disable old features and people could upgrade at will.
Except that there's no business case for rewriting old code that still works and is still supported by the compiler. And then suddenly, the compiler team has to support multiple, very different versions in the same release.
With 10 years between each such 'mode', and the differences still being relatively minute, it would take centuries to morph C into something else. Which is a good thing, don't get me wrong. It just shows that the proposed approach isn't feasible.
Backwards-compatibility is held extremely dear to the C community, with those few breaking changes being simple (usually) to work around. If you want to break compatibility, you have a better chance calling it a new language.