Proprietary languages work when they fill a niche. I work with the Q/K programming language. It's closed source and, from what I understand, has a significant l

Proprietary languages work when they fill a niche. I work with the Q/K programming language. It's closed source and, from what I understand, has a significant licensing fee. But it is a language designed specifically to power trading systems (via the KDB+ platform/ecosystem). It fills a niche well, and is popular in the industry.