You underestimate the advantages of incumbency. YouTube is arguably the lowest common denominator in terms of quality (eg most Filmmakers prefer Vimeo because it gives more control over quality levels), but that hasn't stopped it being #1 by a mile. Getting a critical mass of people to shift to another platform is pretty hard.
I think I'm at least partially on YouTube because of YouTube rather than the content. I pretty much only watch videos on my phone, and it the only video app that I have, and it's good enough.
But there's also no guarantee YouTube will retain its viewers.
The viewers are only there for the content, not for YouTube.
If YouTube doesn't have the content, it won't have the viewers.
This is a market segment that can shift much more quickly than people (including Google, apparently) imagine.
Content is still, and will always be, king.