For literal decades they proved how malleable the protocol is, yes. Then they decided to ignore how malleable the protocol is instead of even trying to e.g. start by deprecating obsolete functionality and disabling it in future versions.

When XRender was introduced, for example, was the perfect time to deprecate server-side font-rendering. It's trivial to shim on the client side if anyone cared about the legacy functionality, and it takes a trivial amount of code to switch to using XRender instead for it instead (been there, done that, written a font renderer).

There's been plenty of opportunities to gut the legacy parts of the protocol that way, and reduce the complexity.

Grouping the main set of extensions and declaring that if the server reports a certain version number or above they

This didn't happen because redhat/ibm/microsoft took control of the project and decided they wanted to kill it. They have said this directly on social media that this is their explicit goal so it's not as if this is a conspiracy of some sort, it's just their publicly stated goal. They then deliberately stopped merging changes and left it to rot until the contributors got frustrated enough to fork it.

Now a massive gamergate-esque coordinated character assassination campaign against the creator of xlibre is ongoing even from publications that have many previous articles praising the same contributor at length. Corporate control of open source is very dangerous to the ecosystem as they also fund the "journalists" writing these articles who are willing to defend every decision they make.