Erlang's a half day for onboarding if you're onboarding a Haskell programmer, not because Haskell programmers are that awesome but because they've already had to learn the hard things involved in Erlang to work with Haskell [1]. Otherwise, that's ludicrous nonsense. Most programmers will need several hours just to figure out how to write the equivalent of lists:map in the new Erlang, immutable world, more hours to figure out how to write things non-trivially with it, and a good couple of weeks at a bare minimum to figure out how to structure applications in an OTP world, then some more time to figure out how to debug it.
It's not especially harder than learning SQL or the latest Javascript framework or other things that aren't Algol-descended languages, but it's nowhere near that easy.
[1] I went the other way. I found Erlang an excellent introduction to Haskell; you get to absorb some of the challenges like working in pervasive immutability in an environment where you're not also swallowing a complicated type system or strict effects separation.
Compare that to Erlang's half a day to a day for onboarding, and one to two weeks before pushing stuff to prod.
I can't imagine any company that could afford three full-time weeks of onboarding.
> investing 3 months for life-long increased productivity (even if it's just a few percentage points; most feel the effect is larger)
The tales of increased productivity are purely anecdotal. Especially if you're talking about a few percentage points.