I'm happy to be involved, but I think the current state of Ammonite is that it needs more maintainership before we can even think about making it standard.
Although I think I'm doing a terrific, amazing job maintaining it (lots of people are talking about how good a job I'm doing) I think that including libraries with only one maintainer as a "default" experience sounds to me like a recipe for bitrot and eventual abandonment.
Also, this is unfortunately incorrect:
Ammonite works on all platforms I think
While Ammonite scripts should work on all platforms, the REPL unfortunately does not https://github.com/lihaoyi/Ammonite/issues/119
While I haven't used windows for programming for a few years now, it's probably pretty common in the overall community. Unless someone makes this work, Ammonite wouldn't be able to serve some large portion of Scala's user base