Finding all the intricacies and gotchya's of a new framework does not happen in a couple of weeks. Also, a total workflow change is not adapted to in a couple of weeks.
"you don't always need to think about the ability to hire additional developers who know your framework of choice"
Unless you are hiring a junior dev who has no experience I think this statement is pretty far fetched. The ramp up time for a new dev on a framework that isn't widely adopted yet will be way higher, plus hiring developers who would even want to work with said framework sounds like a task in itself as well.
If you want to enhance HTML use Vue, if you want to focus on Javascript for everything use React. I like the idea of Svelte coming in to push other frameworks forward, but by no means do I think Svelte will be the future (this may not age well, but at this time my opinion).
The people that run the voice program at Mozilla seem to be very disconnected from the rest of Mozilla. They are essentially an "innovation team" that jump from project to project across the company acting as almost an agency inside Mozilla. Seems this model has really made projects like this suffer because there isn't a team always there to iterate on the project and make it better.