Failing fast isn't easy if you're also learning a new language. You want to go from zero to product as soon as possible, and that means using a language you already know like the back of your hand, even if that language sucks.
Because my experience is, as I related above, even though I was learning elixir (for real, I had dabbled with it before) an building a team from scratch, we delivered a product extremely fast.... because we chose elixir.
It doesn't matter how well you know java, the language is going to slow down your development dramatically... at lest compared to Elixir (and maybe go.)
I think you're also over-estimating the time required to learn a new language to be good enough for most startups. I understand that there are some areas where language mastery will be tremendous benefit. In that case, I agree that switching languages may not be such a good idea. But, that's a special case, IMHO.