Dart is a really simple and ergonomic language to pick up, and there are all sorts of hobbyist languages and frameworks that have niche developer interest. It doesn't have to replace anything major to survive.
> Dart is a really simple and ergonomic language to pick up
Are there any major projects besides flutter built on dart? The fear is not that dart is hard to use, it's that it would not be maintained if flutter was not a mainstream success.
A quick browse shows a few notable names like Square, Tizen, and Toyota. Amazon seems to have also forked Flutter themselves to make their own framework so expand Flutter to Desktop and Web: https://docs.amplify.aws/
Seems to be enough buzz around it that I wouldn't be super worried.