I think it would be more along the lines of:

"if all the existing volunteer mainteners for this language went down in a single airplane accident, would businesses who depend on this language step up and fund its maintenance and could they support that organically from the margins of their business?"

When the answer is yes you have a language that is going to live forever independent of whether it's a fad this year or not etc. Which is partly why I think some seriously out-of-fashion languages keep going surprisingly strongly. Meanwhile some pretty awesome languages just wither on the vine.

Not sure why you're getting downvoted, you have a point. Seems like a "no true Scotsman" fallacy:

"Yes they're used successfully in some companies, but they're not truly commercially viable".

anything can be used "commercially succesfully" given people putting enough work / being bright enough to use the tool. It's not a very useful metric taken like that.

Not really, too much work and the business will go under. Imagine Zuckerberg coding the first Facebook in ASM. It most likely would never have gotten of the ground.

it's not a binary question, it depends on who is asking. If someone is asking who has limited resources or time to invest and not access to a lot of engineers then the realm of what's commercially viable shrinks pretty quickly.

If you have someone who has the resources and time to experiment, then sure everything that is literally viable may be viable.

In general, it's probably somewhere in between. I wouldn't take one or two companies or even solely proprietary companies as a good example though. For the average person it probably matters most if a language has a rich and alive open ecosystem.

I was thinking commercially viable in this context meant for the dev learning it. If you learn Haskell what's your chance of getting a job with it as opposed to any of the other languages you know?