That may be true, but you are assuming that the only other option would be to draft a quarterback. What if the new coach that comes in brings in a veteran quarterback?
Also, whose to say that a new coach may keep Dak for one year while they prepare a quarterback they draft this year?
I'm not saying that will happen or that it is even likely but rather it is smarter to make that decision after you hire a new coach, not before, if you have the luxury of doing so, and the Cowboys do.
Even if the new coach comes in and Jerry tells him, "Dak is our guy", at least it will appear the new coach was on board with that idea to the media and fans. Otherwise, all that would ever be talked about in the coming years would be how "Jerry forced Dak on the new coach".