Social networks don't succeed or fail on casual viewers alone. Youtube is a video sharing site, not a content producer. If they get so toxic that the content producers start finding alternatives, then the casual viewers won't all leave right away.

If it gets so bad that big creators, like pewdiepie, have alternatives that grow in relevance and youtube loses its critical market share then it will eventually lose the casual viewers too, especially if those alternatives aren't up to their eyeballs in ads.

We saw this with digg losing its place to reddit, where they sold out their content to publishers. Content got thinner and worse until the vast majority of users left for reddit.

This may not be the straw that breaks the camel's back. For reddit it was the API lockdown, for twitter it's... well I could point to any number of individual decisions but let's just call them Elon Musk. Facebook hasn't quite hit that tipping point yet I don't think.

With youtube I can easily see this being part of a string of decisions to promote publisher content over user content. They're already selling views which could really sink them in the end.