Companies are doing the baseband processors because that is the easy part. Buy an Arm core, which is what everyone wants to use, integrate some peripherals and you can offer something. The major companies are doing their own apps processors but they are having problems with the modems. The hard part is the complexity of the modem and the software protocol stacks that go with it. You can find companies that will develop a protocol stack for you but don't expect it to work as advertised.
I think many of these baseband IC companies you are spotlighting, plan to get some hardware ICs out there and hope somebody writes some software to go with it. Good luck with that. You're going to be staring at your pretty hardware ICs for a couple of years.