I checked out Splice and they are using Angular/Rails for their web app? Both builtwith and HTML source code for their marketing website and logged in app indicate this.
I watched their talk video [1] and they seemed to have only built an MVPish app using Flutter (ie, less than a year using it + a team of two devs). And it only seems to be for the desktop app?
Maybe I'm missing where they are using it on the web... but I wouldn't call this a huge buy in.
A killer app is something like Rails. Rails made Ruby popular, because it was so good that people who otherwise wouldn't have been interested in Ruby were willing to use it for the benefits they got from Rails.
I suppose Flutter is Dart's killer app, since you're comparing frameworks and languages. Rails is to Ruby as Flutter is to Dart, as it were. I suggest you give it a try. Personally, this felt like the "killer" feature to me, fully advanced animations that I'm not sure are possible on native widget based frameworks: https://www.youtube.com/watch?v=FCyoHclCqc8
Splice is now on flutter, including web. I'd say that is a pretty demanding app.