This is what people don't seem to get about it... the Chiefs deal was that much better. What do you think the Cardinals would have offered at best? A third round pick? Maybe their 2nd pick? Yes, the Bills had to move back 17 spots, but what would you rather do? Slide back 17 spots and add another 1st and a 3rd, or one measly pick?
The Cardinals couldn't just "give less," when there was an awesome offer on the table.
Edit: Plus we're glossing over the PR disaster the team would have had on their hands if we did hand over a boatload of picks for Mahomes at the time. A team still presumably in contention, with a few glaring holes, with a draft coming up where the QBs were supposed to be better the following year? We probably never would have heard of the Chiefs offer, and would wonder what the Cardinals were thinking.