I’m not sure if I understand your argument at all. I’ll respond to your four points:

1. Why would having more supported languages make it harder to find programmers? On the server, developers can use a multitude of languages (Java, Python, JavaScript/Node, PHP, Ruby, to name just a few). That means that many more programmers can write server code. This article has a whole section “Will Dart programmers be hard to find?” In it, companies that use Flutter report that it is “much easier to hire” developers.
2. This makes no sense. In Flutter, devs call Java libraries on Android all the time (just like they call Objective-C libraries on iOS). And Kotlin uses Java libraries too.
3. Google is a huge user of Flutter/Dart (as well as Kotlin and Java).
4. Again, this makes no sense. Did Java kill C++? Did Python kill PHP?

You ask “Does this approach make any sense?” Absolutely. Are you saying you would prefer it if developers could only use Java for writing Android apps? Quite a few Kotlin developers would not like that at all. Same situation on iOS with Swift and Objective-C.

Dart has the advantage that it can be used to develop fast, beautiful apps on both iOS and Android. But there are still other advantages for Kotlin and (even) Java. Developers are smart enough to pick the right tool for their particular job.