I have to disclose my potential bias here because I am these days very involved in the TypeScript world (with Michigan TypeScript). That said, I would never hide the fact that I was once a die-hard Flow person that.. died hard, haha. (and I'm very glad I did! more on that later)
In the very early days (in every circle I was in) Flow was ahead by every measure (known examples, excitement, adoption, features, speed, etc). You could point to real live running apps like GitKraken and Discord (hopefully my memory isn't failing me on those) that where shipping Flow very early. And you could use JSX! At the time there was literally no known project (especially OSS) actually shipping TypeScript. Not from Microsoft and not from anyone else. It was hard to tell if TypeScript was more of "embrace extend extinguish" (obviously with hindsight we can be absolutely certain that it wasn't that (unless we're talking about a decades long "long con" lol))
But there's something I could never have anticipated at the time that's more important to me today than any feature of the language:
The TypeScript team have done a breathtakingly amazing job of carrying the project through the years. TypeScript, as an open source project, is an absolute triumph: and I think it has everything to do with the people behind it.
At this point TypeScript is "good enough" in every way that really matters. If they consciously stopped developing the language and just focused on performance and bugs I think we should all call that a success. I really believe it's been handled _that_ well.
Meanwhile Flow went the other direction and there were bugs (in the 0.30 era, approximately) that had people pulling their hair out that were caused by the impedance mismatch of them making it for Facebook first and the rest of the world last. As far as I can tell, this really never changed (or at least, if it did, then maybe it was "too little too late").
Meanwhile, it seems like TypeScript has been managed with the literal opposite mindset: putting us in a situation where only one major project is left using Flow... React.
There are other projects like ESLint that have been... how do I say this in a not bridge-burn-y way... surprisingly reluctant to realize that TypeScript isn't some passing fad. But I'm confident that they're going to have their wakeup call sooner or later. The culture of JavaScript-land is just not loyal enough for gatekeeping a technology like TypeScript. Look at webpack. We all spent nights and weekends for years gaining knowledge on how it works and weeks per year getting familiar with it, then esbuild happened and we threw webpack away overnight like it never even happened. I believe the same fate awaits projects that deny the necessity of writing in TypeScript at this point in history, with the possible singular exception of React.