I don't understand what you are proposing. If the modal can't cover the like button, how would you implement a modal? How would you implement the standard background fade when a modal is open (which is a div covering the entire page)? How would you propose implementing this change such that it doesn't require millions of websites to change their code?

A full background fade is an aesthetic thing at best, and it would be disingenuous to fully fade a page if some parts are not in fact blocked (i.e. remain clickable). So I think it would require no changes, it would just look different.

Having said that, I see no reason to support Like buttons OR modals and I would be fine with millions of sites being forced never to use them. We know Facebook Like buttons are over-engineered trackers whose domains are best blocked at your router. And when modals are not being abused for things users don’t want, the remaining “good” uses for modals would still be better implemented as less-intrusive and more-asynchronous things (display a modeless background message with buttons, for example).

It seems to me that you've never worked at a company with a marketing team.

The real consequence would likely be that such buttons could not use iframes anymore. Which would probably be good thing anyhow - in the long run.