"Healthy ecosystems" is a disanalogous concept here.
"Diversity" is good in an ecosystem when you have multiple species that complement each other, forming a trophic chain (like plants, herbivores, carnivores, and decomposers).
What you're defending is having two or more species that compete with each other, which, biologically, is a scenario that results in a single victor: https://en.wikipedia.org/wiki/Competitive_exclusion_principl...
The resultant monopoly is good at least initially because the victor, ideally, was the best at the niche it competed for. Having suddenly no competition allows it to stagnate, though, compared to if it had constant competition. Biology solves this with constant, random mutations in a cycle of death and reproduction. Maybe that's what WG21 represents for C++? ;D
In general, though, when you're "trying to do a narrow thing", diversity seems bad all around. Python beat Ruby because it was the totalitarian "there's one right way to do a thing" alternative, which was better. Apple stuff works better because it's a totalitarian design from the hardware on up (I admit this as an Apple hater). Cargo works better than CMake because it's an integrated and opinionated build system.
The "narrow thing" could even be a political agenda and a particular subset of social interactions, in which case diversity can also be viewed as negative politically and socially. See political gridlock and how much strife exists between races, religions, classes, and genders. For example, take the "don't go alone in a room with a woman" spiel that OpenBSD guy blathered about. He seemed to claim that women of low status with political axes to grind would seize clout by claiming false sexual harassment in situations without witnesses. If we suppose that scenario exists, it would impossible were there no diversity of gender/sexuality/politics/status in those environments, which would be better for the "narrow thing" of "have a BSD conference or whatever with no drama".