But if that is impossible or impractical for some reason in this
particular case, then it simply means that users of Emacs 29 will be
unable to upgrade Eglot without also risking less stability due to
upgrading the dependencies. Again, if you don't think this
"dependencies hell" is a Bad Thing in general, we will have to agree
to disagree.
No, it doesn't mean that, at all. First, dependencies exist. And here
it's not hell at all, not IME. Secondly, stability is a matter of
expectations. Eglot users_expect_ :core dependencies to be upgraded
when they request Eglot to be upgraded. That's the way it has always
worked for ~5 years. And non-Eglot users_also_ expect that, because
installing any non-:core ELPA package that depends on :core packages
has_also_ always produced that behaviour for dependencies.