I disagree that there is no room for PL innovation left. I think the situation with Go is that the authors were deliberately trying not to innovate too much in the interests of familiarity.
There's lots of programming language innovation going on, you just don't see it in languages like Dart and Go. Which, by the way, is fine; Go has plenty of reason to exist without being innovative from a PL standpoint. (I would point to languages like Clojure, Kotlin, Scala, and C# as examples of industry innovation in programming languages.)
I guess this is sort of flame-baity, but what did C# really innovate in? Even the highly-lauded LINQ is a few functional features + reified code (and rather ad-hoc at that). I know LINQ is award-winning, and probably changed the industry by putting functional concepts in front of tons of people that wouldn't have otherwise used it. But is there anything even remotely new in C#'s actual language design? (Yes, anyone can be a critic.)
Async workflow was in F# about 5 years before C#, and implemented purely as a library - no hardcoded keywords needed. As I understand, any language with monad syntax can create such a feature.
The new generic variance is actually interesting. AFAIK, the MSR team had that as part of the spec, and it's been sitting in the CLR since 2.0. It's curious that C# is the only language besides MSIL to expose the feature.
There's lots of programming language innovation going on, you just don't see it in languages like Dart and Go. Which, by the way, is fine; Go has plenty of reason to exist without being innovative from a PL standpoint. (I would point to languages like Clojure, Kotlin, Scala, and C# as examples of industry innovation in programming languages.)