I completely and undoubtedly agree with everything you are saying, but just because it hasn't been done before doesn't mean it can't.
We are fully aware there will be a day where we have to stand on our own tow feet, and we will be ready when that day comes.
Option number on is we release a free version with limited capabilities and a upgradable version from $.99-$4.99. This isn't the desirable route because the more users we have on our network, the better it will work. We want to encourage anyone and everyone to join.
Option number 2 (the route we will take) will mainly consist of monetizing businesses who want to use the platform. Much like Snapchat, this will consist of sponsored content, geo options, and other business-centric features. This option will require us to scale quickly, but I am pretty sure it is the route we are going to take.
For the first few months we are going to bootstrap it and focus on user experience and not focus on monetizing users at all. As we scale and grow our user base we can start focusing on things like that.
First, there was ICQ and AIM. ICQ got bought by AOL, and they added compatibility between each protocol.
Then there is Pidgin (formerly known as GAIM) for desktop OSes (Windows/Linux/macOS). Adium is a macOS version of Pidgin, and Bitlbee is an IM to IRC gateway fork of Pidgin. Which means you would be able to access it with your favourite IRC client (possibly with the aid of Tmux/Screen). There were tons and tons of plugins for all these applications available, adding all kind of features, including adding of protocol support. I remember even different versions of protocols being supported. There was even a Skype plugin for Pidgin.
But Skype didn't quite like that plugin. Which brings me to a point: you may end up with fierce opposition from the developers or owners of the official clients, for a myriad of (valid & less valid) reasons. Using Pidgin with ICQ meant less advertisement income for Mirabilis/AOL (the ads were shown when a user would message another, in the message box). Skype/Microsoft doesn't like usage of unofficial clients. Moxie Marlinspike wants people to download the official Signal client from Google Play, and he doesn't want third party clients to use the Signal infrastructure. WhatsApp sued the maker of an unofficial client. Bnetd got taken down [1]. Unofficial KaZaA clients. Really, I'm digressing from IM now. The stories are endless.
Then there is Jabber or XMPP. This had backwards compatibility with previous protocols. Then there were clients which were basically a frontend for those protocols. An outstanding one was Nimbuzz.
[1] https://en.wikipedia.org/wiki/Bnetd