If the open source baseband was even remotely feasible to do, open projects like Openmoko, OpenPhoenux (GTA04, Neo900) together with OsmocomBB would already come up with 100% open GSM device. The people working on those project dream to be able to do that, but they simply can't. OsmocomBB is practically a research project, as there are no practical use-cases for it to "normal user" (in most countries it's illegal to use modem with OsmocomBB on it unless you're operating it with your own BTS-lab network you got permission to set up for development or research purposes), and it only operates on very old devices with TI Calypso, as basically all of more modern basebands are cryptographically signed (TI Calypso was also supposed to be, but for some unknown reason that feature was disabled, probably due to misconfiguration at the factory - this is the only reason OsmocomBB was possible at all).
Unless we do lots of legal lobbying and raise much more resources than a company like Canonical has (trust me, building proper 4G modem is awfully hard and expensive. You have to comply to several thousands pages of protocol documentation and pass many certifications. Canonical probably could would be able to afford producing Ubuntu Edge, but they certainly won't be able to afford the modem development), it's much more helpful to look at projects like Neo900 ( http://neo900.org/ ) which aim for the best possible separation between APE and the baseband with built-in monitoring in case you suspect modem might be doing something malicious. In my opinion, this is the proper step forward the truly free mobile devices in our pockets, not shouting and demanding open basebands (even if we all, including Neo900 developers, dream about them).