To me it comes down to usage: if given a problem X you are likely to chose language Y to solve it, then Y is viable. That sentence probably sounds super obvious, ok. The important point is realizing that the critical question there is: Am I likely to chose language Y? And realizing that the answer doesn't depend on popularity. For a general-purpose language popularity is probably a big factor, but for other languages that shine in solving particular problems it might not be very relevant. The question, as I say, is: is there any scenario where I would chose this language over other alternatives? If the answer is yes, then I'd say the language is viable.