By stagnation I mostly meant the comparatively tiny ecosystem of libraries vis-a-vis, e.g. JVM-based languages. In particular, with Akka, Scala has much nicer support for concurrency. The ability to call, and be called by, Java code in the smoothes possible way is also beneficial for my use cases.

I'm surprised that GADTs were added, because a few years ago I visited the Gallium team at INRIA who develop Ocaml, and asked about GADTs. I was told by one of the senior Ocaml developers that there were no plans for the inclusion on this feature.