I consider two things to think Peertube not being sustainable isn't the case.
First, the noise caused bad actors / professional fearmongerers, and people too used to Youtube or that think any social medias would skyrocket in the first month of service, may make people think it's a far more prevalent opinion.
Second, platforms such as Peertube may cather to any movements, be them cultural, political, for business, and so on, while also, due to being based on instances, it much harder to be taken over.
Those two together make me see the project as having great potential, a potential that some may fear intentionally or otherwise.
And on a side note, "the new mobile app" reminds me, anyone could potentially make programs for it, or even integrate Peertube to their own. Another reason for it being able to cather to way more people, I think, as then programs could be made to interests and needs otherwise not found.