Other than isearch, most other commands should (ideally) read their
regexps interactively with `read-regexp`, so it should be easy for
a third party package to advise `read-regexp` so it accepts the PCRE
syntax (or the RX syntax, ...) and then converts it to Emacs's
own syntax. Shouldn't break any package at all.

This is what `pcre-mode` does in pcre2el https://github.com/joddie/pcre2el/blob/master/pcre2el.el#L707

The package pcre2el is already quite satisfying for me, but given it requires a bit of updates to work and is always a bit kludgy due to having to fight emacs, I would like for Emacs to give better PCRE support so packages like this become obsolete or really trivial to write.