You make good points, but I still think what I envision would be able to attract enough people interested in specific hobbies, without achieving anywhere near Youtube's scale. I'm thinking of a scenario where the video platform is more an extension of a web community, such an an old-school forum, rather than a straight video host where the primary aim is to gain any engagement whatsoever, and where (let's face it) all engagement is generally fungible. It'd be something member-funded and run, like good torrent trackers, and the content is an interest 'ecosystem' - so not only fishing content, but fishing gear coverage, and camping and hiking stuff, and meat prep and storage, and boating, etc.

This couldn't be any worse for either creator or viewer than what YT subjects them to. There would be no having to optimize for an opaque algorithm. The pressure to self-censor would be greatly relieved. Monetization scope and content guidelines would be accountably managed - ie. by the community itself. Creators would still have their Patreon/Liberapay/etc income streams. The platform can place the odd banner ad too, like 4chan.

I wonder how much convenience and (perceived) income security is a passionate creator prepared to sacrifice in order to start exercising power over Youtube by uploading elsewhere? We all know creators hate the place...

Yep, my entertainment is 90% YouTube and the rest some show. On YouTube I find everything: from a dude that does reviews of air filter for cars to somebody explaining some obscure Japanese woodworking techniques to the omniscient Indian dude that explains complex programming concepts. If there was fragmentation I wouldn't be even able to find stuff, like in the early days of the internet that you knew the website existes because somebody shared the URLs in some usenet or some forums, before search engines became a thing.