I had a similar idea for a while and was thinking of creating one.

Yes, most communities set up a Discord, in other words they depend on it or see a need for such application.

Instead of sending HBD, a customJSON is lighter, but it still requires RC credits.

The question to ask is: what is required from a messaging app. And the number one feature is message authentication. In other words, if anyone can impersonate any user, the app would be unusable.
Now, to achieve authentication, key signature is sufficient as long as we know that the public key belongs to given person.
In other words, there is no need for the chat messages to be stored on chain.
On top of it this can be done in decentralized manner.
How? Let's say there is a chat server node. Anyone can run an instance of it. There can be public nodes that are willing to store all community chats. If someone runs at least one public node, the general hive user can use it, or community owner can use it.
So community owners do not need to run their own nodes, but they can. Now someone might run a chat node that is private, or limited to a list of communities. Furthermore, each community owner can specify if their community can be accessed with public nodes or only a list of whitelisted nodes.
This way you have a decentralized chat app. And you can achieve the following two use cases:
Some people would like some channels history to be saved for as long as possible. (use public nodes, if no public node stores chat log for long enough, run your own) Others would prefer to have it deleted after x months. (if you use public node it might "not be honest" and store your data for longer, thus run a private node)
Or public nodes could store chat logs for X time / Y size per channel and ask for a small fee for permanent storage.

Today I submitted a proposal to receive feedback for such a system https://peakd.com/proposals/224 and now I just found your @taskmaster4450 and @edicted post.