The days of Instapaper and the like where you could build a really good app and sell it, are long since past. Nowadays I'm not sure it's really possible to make money as an app startup. Your best bet is probably releasing a game and being the one-in-a-million that goes viral.
Other than that, apps are the purview of existing big companies, and those mostly are concerned with lowering costs (since margins on mobile are so thin). With that being the case, something like Flutter that gives you a fairly good cross-platform experience are going to thrive.
It's sort of like Java in the enterprise. It doesn't matter if it's pleasant to use. There's a clearpath towards success, it's safe for cheaper developers to use and not screw things up to bad. It will never be the hot thing but it doesn't need to be; that's not its target demographic.