You don't need to imagine it, you can just look at what MS has done with .net and the community that's forming as a result. Java has such a horrendous ecosystem, it's really unacceptable.
2. What .NET open source community? I'm not aware of single significant open-source community project in that ecosystem that isn't a lagging imitation of something from the Java community (e.g. Akka.NET, MassTransit, etc). There's a lot of excitement around VS Code, and the possibilities of future stuff on .NET Core, but chatter on HN and Reddit isn't the same as actual shipped code.
3. If by "ecosystem" you mean web forums filled with college students chatting about it, then fine. But I think for most people, "ecosystem" refers to the availability of production-grade libraries and infrastructure components. Try naming one server-side niche in which Maven Centrals lags behind any other language in existence.
This has not been my experience at all. Java's ecosystem and vast libraries of battle tested software is unmatched on any platform.