Modernizing that project would be a nightmare. It's maybe the code base with the most plug-ins, and they should be supported while deprecating and gradually phasibg them out.

I'm working with a client that's built more code in custom plug-ins than Wordpress itself, I'm ambivalent on them not using a more modern code. On one hand it's hard to work with what Wordpress is right now, on the other rewriting all of those plug-ins is a monumental task.

But yeah, at least use git.