[cryptonector]

If you include DSLs in what you say, then I disagree vehemently, at least as to DSLs.

For larger programming languages, it depends. Mainly it's a very long-term commitment. It requires having a strong commitment to funding a team to develop, maintain, and support it. Is that a good idea? I don't know in general. In specific cases it may be an absolutely brilliant idea, though I suspect in most actual cases it's a terrible idea mainly due either to inadequacy of the business case for the proprietary language, or inadequate funding. In order to build and keep proprietary a programming language for decades the value proposition in it has to be enormous -- that makes the business case unlikely, but not impossible.