If they move components to enterprise in a new version, how hard is it for the community to fork the last open source version of those components and port them to work with the newer version?
That has happened several times already. The best known fork is probably Tryton, which forked during the OpenERP transition. It has seen a lot of independent development since then, also with some sort of company backing. (I evaluated it many years ago so my experience is probably no longer relevant.)
It would take an experienced odoo Dev to fork and maintain it. And they would also have to take the huge amount of business requests and support questions.