I think you highly underestimate the amount of people who exclusively use their mobile phone to communicate. WhatsApp can only be used on one device - one single mobile phone - at a time, yet it has a billion active users. If desktop clients exist, lots of people will use them. If they don't, lots of people won't truly care. In fact I just realised Monal actually has a version for OSX, so there's that. If your point is that the clients on different operating systems have different names, I also think that isn't making the situation easier (because people will have to figure out the name of what they need to install depending on which kind of phone they have), but that's almost inevitable with a decentralized protocol. Almost (this has a lot of potential to be the next big thing).

Conversations and Monal

All of those work perfectly fine in Conversations, I'm pretty sure in Monal as well but I don't have an iOS device to confirm.