libpurple (the actual messaging library) is already modularised, many different people mantain[ed] different modules, it's been used my many third party messaging clients including Adium and multi-IM browser clients from those days like Meebo or imo.

It's not a commercial project with a CLA, it doesn't have a single owner, many of the people who contributed major parts have moved on or would object to a license change from copyleft to permissive, so it's not a matter of "just change" the license. And libpurple is the real meat of the project.