I think what's missing from this post is who is using a language. For Haskell it's academics/researchers. For Ada it's the US government and associated industries. That's what makes niche languages viable, not overall popularity.

Of course if a language does not have a niche nor popular support then it's going to struggle more.

> For Haskell it's academics/researchers

Some real businesses use Haskell too.

Other than that, you have a point.

Most known languages have a few businesses that use them. That does not mean is enough to be commercially viable, though.