That's true, but as long as the baseband doesn't have access to the rest of the device, there is at least a limit to how much damage it can do, which is a good

That's true, but as long as the baseband doesn't have access to the rest of the device, there is at least a limit to how much damage it can do, which is a good incremental step. If it can't exploit the OS, then worst case, it has access to network traffic (just like the next router in the chain, which is why all the traffic should be encrypted), and it has the ability to help locate the device against the user's wishes (but so do the cell towers). Rewriting baseband firmware is also a great opportunity to make it Open Source, and I hope that happens as well.