We had Trillian back in the day, but then the messaging services locked down their protocols. I don't expect such an app would survive the cat&mouse with major players for long.
MSN/WLM, AIM, YIM, ICQ, Gadu-Gadu, Skype… all of them were reverse-engineered. The only open protocols it supported were IRC and Jabber.
The European Union is also working on breaking open the messenger space, forcing tech companies to either leave the EU or work on an interoperable standard. I believe an IEEE working group is already developing a protocol to serve this purpose, even with encryption available if I recall correctly.
I believe this is the relevant draft: https://www.ietf.org/id/draft-mahy-mimi-identity-01.html
Edit: I should add that as far as I know, the working group hasn't done much as of yet. I heard about this on Matrix because some of the Matrix folks are trying to get (parts of) the federated Matrix protocol into the standard, or probably at least parts of it.