SEO and "algorithms" like YouTube are tuned for maximum user engagement, which means people watching more videos for longer. That's a pretty decent proxy for enjoying what you watch.

variety is way down in the more successful youtubers

I honestly don't watch any of the top YouTube channels. I don't sub to any of these, and I don't think I've watched a full video of any of them, aside from maybe a few music videos. In fact, the channel with the most subs that I sub to is JerryRigEverything, and I mostly sub because he's local (we're both in Utah). Here's a selection of channels I sub to that I watch the most:

• Gamers Nexus
• Digital Foundry
• Louis Rossmann
• Stand-up Maths
• The Money Guy Show
• Naomi Brockwell TV
• Level1Techs
• Audit the Audit - starting to do more clickbait titles and thumbnails, but quality remains high
• The Phawx
• Leo Vader
• Mental Outlaw
• Optimum Tech
• Mr. Puzzle

They do a pretty good job of keeping titles informative instead of clickbaity and thumbnails relevant (I honestly don't care about thumbnails). I have abandoned a lot of channels because they do that nonsense and don't go as in-depth as I'd like, such as SomeOrdinaryGamers, LTT, and Jayz2Cents.

I honestly recommend any of those channels, they're about all I watch on YouTube. Oh, and only 3-4 of those have sponsors (GN, Stand-Up Maths, and Audit the Audit come to mind), and very few of them ask for subs and likes. I block ads and occasionally buy merch from them.

Edit: Forgot Tech Ingredients.