I think TS had better tooling and implementation. Flow continues to improve and had some better design decisions early on. Flow has focused on internal FB needs over open source needs.
All in all, I think the soundness is not a negative / defining character and Flow could still gain in popularity, especially for projects closer in requirements to FB’s codebase.
We use flow extensively at work (we have a monorepo w/ some 400 projects in them). `ag FlowFixMe | wc -l` gives me almost ten thousand hits. Which means that in practice, there's a lot of unsoundness going on even if one chooses to use flow over TS.
A lot of people at work dislike flow because while it is generally sounder than TS, it throws some really stupid "errors" (e.g. Array prototype.filter doesn't refine arrays of nullables, it can't tell property access isn't a getter w/ side effects so it errs on the side of assuming it always is, etc).
Typescript simply draws the line in the sand clearly from the beginning so that its scope as a project stays well defined and it doesn't end up accidentally falling into obscure gaps like flow.
All in all, I think the soundness is not a negative / defining character and Flow could still gain in popularity, especially for projects closer in requirements to FB’s codebase.