I would argue that becoming "legacy" is akin to bit rot. Sure, it still runs, but everyone that knew how it worked has left the company, and it's written in a language that is no longer in vogue and will require specifically hiring people with that skill to maintain.
As a counterpoint, I think we can both agree that writing production software in one of the toy languages that pops up on HN frontpage every day is a bad idea. Writing in a language that is old enough that it has become niche has many of the same issues. It doesn't really matter whether the software is poorly understood because the language is new enough to be niche or old enough to be niche; the outcome is the same.
As a counterpoint, I think we can both agree that writing production software in one of the toy languages that pops up on HN frontpage every day is a bad idea. Writing in a language that is old enough that it has become niche has many of the same issues. It doesn't really matter whether the software is poorly understood because the language is new enough to be niche or old enough to be niche; the outcome is the same.