While you're tiny, you need discoverability a lot. But even if YouTube bans you and deletes all your videos, you lose relatively little.
The bigger you are, the more well-known, the larger is your following, and the more the whole enterprise is the source of your livelihood, the more you may need to hedge your bets.
It always makes sense to have a backup, but the issue is that video creators are always naturally losing viewers and need to replace them by new viewers who discover their channel(s). It seems like the new viewer discovery is all happening on a very short list of giant platforms which is a bit worrying. Personally I like the way podcasts have managed to be more decentralized but I also fear podcasting is slowly becoming YouTube centric.