I did; none of those messengers are P2P AFAIK, which brings us to the point of this problem.

What do you guys think prevents people from building a convenient, easy-to-use blockchain-based P2P mobile app?

Well, I do know what - the incredible strain any PoW protocol puts on a mobile device and the amount of storage needed for the whole blockchain. So far the only solutions to this were different sorts of "crutches": like making a remote PC be a PoW whore for your phone, which means that you either have to set up a node on your computer and have it running 24/7, or pay someone to do that for you, which isn't what many people would call an "optimal solution".

What do y'all think can be done with a PoW system to make it applicable for mobile devices?