It’s a huge risk if the unit of ownership is “engineer”. It’s less of a risk, but still not zero risk, if the unit of ownership is a squad/team/whatchamacallit, because you’ll get more brains applied to “should we really use Haskell for this?” and are isolated from the one Haskell engineer quitting. (Teams do rarely quit together, but individuals leave all the time in comparison.)

It’s also a risk to force your entire company to use 1-2 languages (one of which is [practically for most] hardwired to be Javascript, so you get to pick the other one).

At some points in time, c, c++, php, perl, or C# would have been reasonable choices for web dev. Do you want to be leading a company of Perl experts maintaining a large Perl codebase right now? Doing it, I can say that even C# is painful when it’s your mono-language monolith. (Inertia in language cuts both ways.)