This is true. But the QB argument is not about what can be done, but what is likely to be done. As others have said, there's no better way to consistently compete than with a top-flight QB. Going the decent and defense route sends you into essentially a 2-year window before the defense gets dismantled due to the cap.
If the Bills sell the world and end up with a HoF-level QB, nobody will care about the price. However, if they sell the farm and end up with Jameis Winston, they won't be better off than having kept Tyrod and built the roster more. It's definitely a risk, but that doesn't make it an unworthy risk.