The difference between this and the plethora of different video services, is that the product here may not be the actual video service, but can be seen as a way to support the various video creators.
Most of the creators on Nebula are already drawing in pretty large followings on Patreon, so it's clear that they are able to retain dedicated audiences that will pay for content. Even as someone who doesn't watch much Youtube, I can recognize a lot of great channels, like CGP Grey, Lindsay Ellis, Crash Course, etc.
Also, I can totally see the advantage of having a service like this compared to youtube for kids. It's a bit scary how fast children are being exposed to the internet nowadays.
All that being said, it's clearly very difficult and risky to start a video service nowadays, but I think this has a slightly higher probability of being sustainable compared to many of the other offerings I've seen.