> Could the reason be that language design is dead, as was asserted by an anonymous reviewer on one of our team’s papers ~30 years ago?

It may not be dead, but it seems much harder for languages to gain adoption.

I think there are several reasons; I also suspect AI contributes a bit to this.

People usually specialize in one or two language, so the more languages exist, the less variety we may see with regards to people ACTUALLY using the language. If I would be, say, 15 years old, I may pick up python and just stick with it rather than experiment and try out many languages. Or perhaps not even write software at all, if AI auto-writes most of it anyway.