Go doesn't have any kind of story for incremental migration from C++ or Java; you are talking about rewriting all those codebases from scratch, which is an obvious nonstarter as long as engineering resources are finite.

IIUC Fuchsia uses Dart mostly for UI stuff and Go has never really tried to be competitive there? I don't see much of a reason to suppose this is a serious bottleneck to Fuchsia adoption, as opposed to the obvious reasons why it's hard to displace an existing OS with a huge install base.