YouTube as a whole has a giant moat.

But niches within YouTube can be disrupted. We've seen it with short form (TikTok etc), music (Spotify etc). We see it with specific niches of content creators (nebula etc). It's happened with livestreams.

I'm bad at predicting future, but could imagine niches like "publicly funded content" from e.g. EU public broadcasters moving away (e.g. NPOstart in NL) because of privacy issues or because they legally can't monetize their content anyway. Maybe university lectures? Or sports video? Game reviews by a specialized platform by steam? Video between 4 and 10 minutes? Podcast videos?

So YouTube as a whole will stay, but it can be chipped away at. Some chips may prove in themselves a small, but still good business model.

I agree with this. For example some tech creators are using peertube or similar. University Lectures now posting in other websites as backup and people that do courses also have them in their website. What I think what will happen it's that YouTube will still be used for discovery to drive the traffic to these other sites until people finally migrate to the smaller ones.