I don't mind SimpleX. I use it myself, sometimes. But it is also currently a very obscure service, and it's not exactly easy to find people to communicate with. Phone numbers, and now usernames, provide a jumping-off point for that.
How do you find people on SimpleX and then make sure you're talking to them in a group later on? Right now, that's really hard.
And right now, SimpleX is pretty small, so if it starts expanding in that first area, how would it prevent spam?