>It’s almost like there are improvements in PL design.

Eh, not really, at least not in the last 10+ years. I'm sure some obscure hotness does something neat, but mostly inconsequential for the vast, vast majority of shops.

>someone will be a senior in any other language as well.

While I agree with you, that often isn't the opinion of people hiring. If someone is looking for 2 years of java, in most places, 10 years of C# isn't what they are willing to hire.