I'm not for sure where I'm missing how this drives Dak's price up.
In the case of Mahomes, he gets the largest contract in sports history, but the Chiefs get to lock him up for the next 12 years of his playing career. It's essentially a lifetime contract. Mahomes has provided an MVP and a Super Bowl to a franchise. But even with his value, to get his deal, he had to sacrifice long term and not being able to go back and dip his toes of the FA water again to get there.
Dak is asking for his cake, and to eat it to. He wants a shorter term deal, and still wants elite QB money. He wants that extra cake in 4 years where he can come back, look at the cap and request an even larger number. Mahomes had to settle for elite money on a team friendly contract length. This from the best QB in the league currently.
If anything, to me, it strengthens Dallas position on asking for a longer term deal. Mahomes had to give up extra years to get his number. Dak wants his number AND his contract length. Best QB in the game moved. But Dak cannot?