A quick specific answer w.r.t. XMPP is that Matrix aims for a very coarser granularity of modularity, published as a single consistent spec, and mandates feature profiles to determine which modules should get used where. In other words, there is only one official set of endpoints specified for Matrix for a given use case (e.g. mobile IM+VoIP). So it ends up being under tighter control (whilst still very much taking proposals and PRs from the overall community), and is easier to swap out the higher level layers to rapidly evolve functionality - rather than suffering the proliferation of XEPs which is XMPP's double-edged sword.

This is always going to move slower than a centralised service, but we think the freedom of being able to select your clients/servers/services is well worth that cost in agility - and does not have to compromise privacy if engineered correctly.