Because of the CORS settings on Google's servers would tell your browser to not go forward with the request. There are two ways it could eventually be possible:
Fair enough, that's interesting. I assume this only applies to the non-web clients. On the web, it would not be possible. You can verify by looking at the outgoing network requests on this random video for example: https://invidious.privacyredirect.com/watch?v=qKMcKQCQxxI
I'm pretty confident that you are wrong.
Invidious and YouTube piped (and LibreTube) by default load the videos server-side, as opposed to GrayJay, NewPipe or Smarttube.
It has advantages (mostly that your IP address is not shared with YouTube, and it allows users from countries where YouTube is blocked to still access it) and inconvenients (much harder to keep up when YouTube actively seeks to block them).
Smarttube next doesn't require rooting the device, it can be sideloaded. Sideloading is not very complicated. Google is not trying to block any sideloading (at the moment, at least).
LibreTube is also a good one. Basically an app for piped
Browsers based on chromium do not have to follow exactly what the main branch is doing. If they want to keep supporting MV2 or support different rules for MV3, they can. Albeit it's a bit cumbersome.
Unfortunately, I think that while ad blockers won't work as well, they will still work good enough that most won't bother making the switch.
https://blog.getadblock.com/how-adblock-is-getting-ready-for-manifest-v3-6cf21a7884f6
https://github.com/uBlockOrigin/uBOL-home/wiki/
https://adguard.com/en/blog/adguard-mv3.html
https://www.reddit.com/r/uBlockOrigin/comments/1067als/comment/j3h00xj/
The main issue I see is the slow update of filters (which require an extension update). This might make YouTube win the cat and mouse game. Where YouTube updates(ed?) their blocking detection multiple time a day.
Firefox's implementation of manifest v3 doesn't come with the same restriction as Google's. Ad blockers will still work with manifest v3 on Firefox (but not on chrome).
This means that all manifest v3 extensions made for Chrome work with Firefox, and almost all manifest v3 extensions made for Firefox will work with Chrome.
They already support manifest v3, but with less restrictions than Chrome's implementation.
For that they use iframes, which have a different security system.