On that $8/month VPS I think at the time I had 2 qualities. 1080 5kbps was the 70+ open streams. I don't expect "big streamers" to join Owncast soon if ever. But if they do, I imagine they have MORE than enough money to be able to afford a CDN or S3. We're not talking MILLIONS per months. I don't think even think we're even talking thousands, hundreds probably. But yes. You make a good point. And sadly it's the point that everyone instantly comes up with WHY folks shouldn't use Owncast. I personally just try to create a welcoming community for anyone interested in trying Owncast. As time goes on those costs of tech continue to go down. If you're running a server from your house, unless you have a datacap, then you don't even need to worry about cost of bandwidth, obviously infrastructure does matter though.
As far as CDNs go, streams are just bunches of files. Your player goes out, grabs some files, and you watch it. So a CDN works for vidoe streams like anything else and I almost guarantee that Twitch leverages CDNs as well.
You could do the same with a S3 bucket as well. So if the CDN is too expensive (I honestly don't know the prices), you could do a S3 bucket.
I have about 12 folks watching me on average at this point. Still better than I had on Twitch. :-) But also, this is MY page. I've tweaked the CSS to make it look more like mine. I can show what I want, I don't have to jump through hoops to keep up with Twitch's algorithm, I don't have to show ads, my page doesn't take 20 seconds to load because it's loading all kinds of junk in the background. I love it personally. It's mine. :-)
If you're interested, just go check out the directory. Go watch someone. See if anything strikes your fancy. :-) https://directory.owncast.online/
I'm not trying to sway your opinion in the least. It's not for everyone in the least. I'm just trying to help folks realize there is a REAL alternative to the big platforms. We ARE talking on an open platform Reddit. ;-)