Switch to PHP plugins?

Problem/Motivation

In #3214707: Provide default examples → I was blocked when I wanted to create examples for theme hooks and render elements requiring:
- additionnal processing: example pager, you need to call the pager service in the code execution
- object variables: breadcrumb and all menu/links related theme hooks

Also in ✨ Integration with form elements Closed: works as designed for form elements. For this one maybe adding a new "form" key will do the job.

So I open this discussion to maybe switch to PHP plugins.

I know that the original goal was to ease themers work but if it is limited and also finallly I wonder if even for themers is not as easy to create a PHP plugin than a YAML one.