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The Linux kernel is also a monolith yet has worked forever without a rewrite.

I think there is a reasonably agreed on set of things that can be removed from X, like server-side drawing primitives, and GLX

The Linux kernel was not born obsolete on arrival like the X Window System was. At least, not fully obsolete.

It will be obsolete someday, anyways.

In what way was X11 obsolete in 1987?

“Obsolete” means that a replacement exists which is as good as, or better than, the thing it aims to replace, in every significant way.

X11 is far from obsolete.