@gamey It won't stop at simply "dropping a DM" on the other end. Inter-operability would also mean participating in group chats/Matrix rooms. So if a single person from WA wants to participate in a Matrix room … do I need to go into detail? If I'd host such a server, I'd block that (I could, as those rules to not apply to me and my "little server).
@IzzyOnDroid I am still not entirely sure what "basic chat functionality" is supposed to mean but you are right. I have even seen arguments about possibly having to accept every server in the group chat indivitually but I highly doubt it will include public rooms. I know many people that I communicate with using SMS because all they have is Whatsapp and their reason against alternatives is a lack of acceas to all their contacts, I wouldn't communicate with Whatsapp directly but they would!