I’m not sure you could kill Matrix now even if you wanted to? Heck, given people still run gopher & WAIS & fidonet servers today, so even if everyone runs away from Matrix, it’ll still keep burbling away somewhere :)

Certainly not kill it, I just don't think it will be nearly as ubiquitous as e-mail.

My prediction is that with further advances in networking, devices will be permanently online and asynchronous communication will become less relevant (because you can always directly reach your peer), so all these fancy E2EE crypto protocols will be replaced with TLS or a similar protocol.