On YOUR "professionnal project". This doesn't apply to all projects. For example, in my case, I had an Android app made in Java with many ugly code (started it as a junior dev), and after two years, I started using Kotlin, then in the past months (has been 1year 1month since I started using Kotlin), I started using coroutines, and it was smooth and incremental. Didn't prevent me from continuing working on this app, which is a professionnal project too, but is only one example. Of course, if you have a lot of concurrency in your project, switching from anything to anything else to manage concurrency can be expected to be hard and long