Matrix: Where are we, what still needs to happen to tackle giants like WhatsApp?

As long as users need to think about homeservers we can't expect a substantial migration.

As far as I'm aware there is work being done on the fully p2p version of Matrix where every device is a server. If that allows abstracting away homeservers, and if they solve multi device identities in a clean way, only then Matrix might be able to compete with an "ordinary" chat app for the majority of users.