> What source would you prefer an open source UI library come from, that would avoid this concern
Note: "this concern" is not "the library may go away or be unsupported". That's true of any open or closed source library.
The concern specifically is "this may get hyped to the point where it feels reasonable to depend on, then go unsupported". And that's never a surety, but frankly Google makes it MORE likely than, say, some random internet package that it will build enough of a groundswell to be seem safe, then suffer a sudden Nest-API-like strangulation. (Convenient for my argument that that announcement came out between my original post above and this one, but it's convenience that is directly my point).
Some rando package is unlikely to be broadly adopted unless it's actually good stuff...and the good stuff tends to get well supported, even if that means the occasional fork. Google has name cachet that can build support quickly, but the track record doesn't support that.
There's never a guarantee...but there are performers that are above average risk, and that's where I put Google when it comes to APIs and libraries.