By your logic the Linux kernel itself should ve replaced since it's decades old and full of legacy code
Eventually, everyone involved knows, it's time. And even Linux has had some major rewrites in multiple areas - don't pretend like Linux hasn't changed immensely since 1.0. Xorg though... hasn't that much...
Heck, there's nothing wrong using anything anywhere, it's a question of fulfilling the requirements.
Meanwhile, you in another comment:
> You can use `xdpyinfo` yourself to see all the extensions you have loaded.
So, which one is it? Is it unchanging or is it evolved through several extensions?