Jones isn't in the same league as he used to be either. In the NFL you pay players for who they are now, not who they used to be if you want to remain competitive. Jones is still really good but it's all downhill from here. I'd take a 24 year old Cooper over a 30 year old Jones.
Besides that reason, Cooper will unfortunately get paid more than he normally would because we traded a first rounder for him. Cooper doesn't have to be reasonable. Similar to the Mack deal, but to a lesser extent, the Cowboys invested in him, and they didn't do it for a partial season. Cooper knows this and will extort what he wants from the Cowboys.