Having worked at multiple unicorn+ SaaS products, the product feature demands are significant.

Yes if you have one sole feature that you can pour all your engineering resources into like whatsapp delivering messages then it makes sense you might get away with 2 Elixir devs.

If you are working at a unicorn who just raised $250m in founding and now has to deliver 100 new features this year... well the elegance of the language won't help as much as you think.