I can't honestly see how any other company can single-handedly stop Google if they go though with this. Google has the ability to strong arm this proposal by having Youtube and Google search dependent on Web Environment Integrity. There are enough alternative to web search but I can't see how anyone can fight Google's dominance in video hosting to stop them.
You would almost have to have every other major website intentionally break on Chrome to even the playing field, and if Google still don't back down you are left with a divided internet.
If all browsers and standards organizations oppose this idea, but Google does it anyway and it succeeds and takes over, can you imagine how easy the anti-trust case will be?
I think YouTube and Google Search are the least of our worries. There will be companies who would have a field day picking up the pieces if that happened.
It's everyone else using it that suddenly means you can't run an ad/script blocker on the ickier parts of the web that really need it. The modern internet is an unusable mess, and only ad blockers make it tolerable again.
You know, most major web servers are open source projects (Apache, nginx, ...). They could in theory decide to check for the browser that's accessing a website and just return an error If it's a variant that supports WEI. Ofc people could fork them and remove the check, but many might just use them as is.
Just a thought though, this would be a very radical and hugely controversial step.