It would eventually give them the smokescreen they want to pull uBlock Origin from the chrome extension store. A while ago they basically used their weight to remove most video downloading extensions and added language to their terms of service which all but says "no youtube downloaders" and I would like to say it cost them, because it should, but most consumers are absurdly apathetic and don't care.
Adblockers are a larger group, but in the grand scheme of things I don't think it will have much impact. The Internet as a whole is a very large group and even though uBlock has a huge audience it's a drop in the bucket.
Unless twitch has some crazy way of inserting ads into the video stream, YouTube will be able to do exactly what twitch has done.
I have never seen any ads on twitch. I just checked with DevTools and it seems like uBlock Origin simply blocks a script called "v6s.js", that is apparantly responsible for serving ads.
There's also posts their daily about how ads are not being blocked on twitch anymore.