Whenever people genuinely get sick of YouTube and decide it's time for an alternative.
It can happen. It took a big push for a certain subsect of individuals to leave Twitter for Bluesky after so many previously failed attempts. (I.e., Pebble/T2, Hive Social, Mastodon, other Fediverse bullshit, etc.)
Because I remember when shit like StoryFire made the rounds on YouTube for a while and that got people really interested, even more so because of some semi-famous YouTubers flocking there to upload their videos dual-platform style, only for the hype to dissipate not even three months later as normies like to keep the status quo intact and 'change is hard'. Also, that project seems to be on its way out anyway.
Some people still use Vimeo and Dailymotion as actual alternatives, but that's likely just a result of them being around for so long and having existed longer than Youtube itself.
Sites like Vidlii are SNCA and unregulated but also still around.
While the websites you mentioned, although niche, haven't had their bell called yet. So that's a good sign.