Where in the intervening time would you have recommended a rewrite? Into which language? The other successful libc projects for linux -- bionic and musl -- are also written in C. Which language should they have been written in? Why do you suppose they weren't?
There may be other reasons it’s the way it is, I’m just saying the language choice made 40 years ago is not one of the valid reasons. People do rewrite things, even complicated things, in different languages all the time when there’s sufficient benefit. There have been plenty of other major changes to glibc in that time.
It’s a component with very, very high leverage. If you improve glibc then you’ve improved millions of other programs. So I would think all options are on the table.