False. XMPP has excellent feature negotiation capabilities.
> everyone using Conversations essentially has to use a server that is maintained by the developers
No, you just need to use a server maintained by someone who actually maintains it.
> Otherwise, most of the XMPP featureset they offer simply doesn't work.
Feature negotiation and graceful fallback, is it really that hard?
> I think there needs to be incentive built into the protocol for all servers to maintain a baseline of XEP parity
Incentives, yes. In the (core) protocol? No. I think publishing a new "XMPP Compliance Suite $year" XEP is better than throwing the entire thing out, in a couple of years when everyone suddenly wants a different feature set.
> At least Matrix can provide a clean slate.
A clean slate which will be obsolete in just a few years, and will have to be invented from scratch all over again.