Well it isn't like XMPP has died or something. There are hundreds of servers and lots of clients. Having the Matrix project doesn't take away from that in any way. If Matrix becomes popular then it could supersede XMPP which is OK.

Right now it is federated vs everything else really. There is no reason to think that Matrix is suddenly going to be used by regular people where XMPP was not. The bigger issue here can't be solved by purely technical means.