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.
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.