Once upon a time, NodeJS, Go and C# weren't battle tested or with support either. "Because we already have one" isn't a great reason not to do something. What this does need is a differentiator over those other languages. Vapor is nice if you already write Swift, which itself is an adopted and heavily used language. Dart isn't, and while Flutter is popular, it's "barely so" relative to the rest of the world. Someone will appreciate this, hopefully enough people to make it viable, but I remain skeptical that will happen, not because of alternatives but because of current entrenchment.
Flutter is becoming very popular for building apps. It is likely to have surpassed React Native, and according to Google more than 20% of new apps are created with Flutter today.
I hear that every year around I/O, but I never see stats to back that up. Have you got any? Flutter has been becoming very popular for years, but I don't know anyone who uses it. More than 20% is a neat stat, but how many of the top 10 or 100 apps are Flutter? How many of those 20% roll out to large audiences? How many don't get rewritten?
Anecdotally, I'm running a Flutter Meetup with over a thousand members in Stockholm, so I feel like I have the ear on the ground. From what I see, the use of Flutter has been picking up steadily. For instance, one of Sweden's major banks has recently moved its apps to Flutter (including its web app). To their customers, it still looks exactly the same, so moves like that aren't always visible. Also, it seems like a very large proportion of startups are using Flutter too.
