If it were shallow, it'd be easy for them to fix

Backwards compatibility, every vim and emacs and bash enthusiast should know about it.

It's easy for the USER to fix, since there are flags available. In the day of LLMs it's also easy to find out about those flags and what they do. And if it's so important, testing shouldn't be supremely hard, either.

Do you mean backwards compatibility for things that rely on the default settings? The defaults have already changed across Java versions and also depend on the system.

Normally when you run a non-Java binary, it uses very little memory to start, doesn't have a limit, and returns memory to the system when it frees things. Supposedly you can set JVM flags to do all that, but performance probably suffers, otherwise they would've just done that. So in practice users are always setting the flags carefully.