> If vendors would be required to release all firmware source code

This may be the hope or expectation you have with regard to regulation, but I don't think it is ever going to happen because it would conflict with other regulatory requirements, especially in the realm of radio.

You may say "regulation can be changed" but radio regulations exist for real reasons and I doubt anyone could convince the major governing bodies otherwise.

Not to mention that such a requirement could effectively force companies to give up intellectual property that they have invested heavily in developing. What if a manufacturer outsources the firmware development to a vendor that specializes in firmware? If the manufacturer were required to release that firmware as open source, they would effectively be forcing that vendor to open-source its product. That could undermine the vendor's entire business model.