I don't know. I guess I just don't see it how you do. It just doesn't make sense to me to build that on a blockchain. Messaging apps that arent built on a blockchain are already about as good as it gets. Most of them you can send payments from as well, or build a simple bot to handle it on slack, discord, rocketchat, etc.
Like I said, also with blockchains comes costs. Most messaging services are free. This is another big problem. How are you going to handle the transaction fees of every single message? Also, what about transaction/block times? Also, what about the security makes it better than centralized services?
I can't think of any real benefit to a messaging DApp. Or at least any benefit good enough for me to switch away from slack, discord, etc.
Payments: These will be Crypto tx; insteading of sending to a string of characters, you'll send to a persona of your friend. How is that not a great addition?
Security: The data is yours.
Tx Handling: Look at Steemit.