That argument applies to something like Linux. Here we are talking about a language and wrappers and an essentially completed framework (granted new ideas mean no framework is every completed).

And unlike a google service, which requires continued resource usage and high cost, flutter+dart has low needs. It may get outdated, but it can't be disappeared like other dropped google projects.

In any case, Unless Dart+flutter becomes an undesirable competitor to another google offering, there is no reason for Google to drop it unless it fails to get sufficient adoption. But I think that's unlikely.

Javascript devs are widening their horizons these days (as opposed to their rejection of Dart 1.0), and Dart only needs a few of those javascript devs to take an interest. The framework is an easy crossover for many of them, with an almost instant access to app development and little commitment needed to spin up.