That's a big assumption.

It's very possible that it's only that profitable at Youtube-sized scale.

There's definitely some very large critical mass necessary for the network effects to kick in. Is that mass such that there can only be one? Or could there be three or four, like other social media? Arguably Instagram is as large a video platform, though obviously very different. Same with tiktok; the individual videos might be shorter but the bandwidth costs scale with watch time.

you don't need youtube scale - but you need to be a lot bigger than most others are. You need to be big enough that ads pay for a full time ad salesperson as well as your other overhead.