Also another development is that the Matrix app on Freedombone now runs on Python 3. This improves its performance and makes it more suitable for running on ARM single board computers with 1GB of RAM. In tests while running a room with 20 users and subscribing to a few rooms on other homeservers, some of which are quite high volume, Synapse on Python 3 only uses 200MB of RAM. So this makes it similar to an XMPP server in terms of resource use.
XMPP still has advantages, such as the ability to proxy through Tor on mobile (the Android Riot app currently can't do that, hence exposing metadata) but the competition is getting closer. Really the idea of competition is the wrong frame here though, because bridging between Matrix and XMPP is improving and so in the end choice of chat software will just be down to personal preference.
Also on the topic of chat systems I noticed that OTR version 4 was announced on day 3 of 35C3. It doesn't support multi-user encrypted chat though, so this is an encryption standard which is dead upon arrival. Yes, a lot of private chat is one-to-one, but in the last few years private group chat has become a major phenomena and to ignore that in your security model is a gigantic oversight. So an easy prediction is that OTR will continue to decline in popularity in 2019.