I disagree.
Have you tried hiring Erlang or Haskell developers? It's not like they don't exist. In fact, there are likely many developers that would love to take an Erlang or Haskell job, but there aren't that many opportunities.
I don't dispute that they exist, I dispute the economics of finding them compared to finding or training new Go developers. Frankly, no one is going to replace their entire team and start a hiring campaign because Haskell or Erlang (arguably) make concurrency slightly easier than Go.
You would be surprised by the result. Economically it make sense. And i would argue that production ready Elixir/erlang is easier to teach than production ready Go.
Still, I'm fairly risk averse. I'll wait for the inevitable explosion of Erlang/etc shops to take off before I jump on board.
Unless you need a massive number of developers, or you're located out in the middle of nowhere, it really shouldn't be too difficult.