> Would you argue that Chez Scheme (to pick a random example) isn't a "serious programming language"?

Commercially speaking? Absolutely not a "serious programming language" in this context.

If someone tries to implement a project using Chez Scheme, they better have agreat reason to not use mainstream boring language.

I used to think like that, but I don't anymore. The most important thing about a project is the people who are working on it. Successful projects are built by small teams with deep expertise pulling together on a shared mission. If they want to use Chez Scheme, Ocaml, Haskell, or whatever else I trust their judgment.