Besides the technical difficulties (trying to reverse engineer the modem internals and reimplement the firmware with no clear reference; likely encrypted and si

Besides the technical difficulties (trying to reverse engineer the modem internals and reimplement the firmware with no clear reference; likely encrypted and signed firmwares; etc) there's a good chance there's also legal issues. If the hardware itself is capable of operating outside of its license (frequencies, modulations, etc), then various certifications from the FCC would likely be invalidated by replacing the firmware and it would become illegal to operate.