I like matrix, but I'm glad they decided to go with SimpleX as an alternative instead of slack, discord etc. XMPP would be fine too i guess, but I think that projects like SimpleX and Matrix need more people to mature, while XMPP seems to have a lot of users but no ability to grow anymore. I also had to choose between simplex and matrix for my personal communications foundation, and I chose matrix as I gambled on its longer existence as indicator for maturity and therefore adaptability for my non-tech friends, time will tell if the bet was right.