Witten signed a 5-year, 37M contract with 2017 being the final year. He have received all of the 37M except for the 7.4M that is his scheduled base salary for 2017. His average pay over the first 4 years was 7.4M ( (37M-7.4M)/4) which also happens to be his salary for 2017.
His 2017 salary of 7.4M is not more than previous years because of restructuring. His 2017 cap hit (12.2M) is indeed more because of restructuring.
The extra cap hit money above his 7.4M salary is money has already been paid and was part of his previous years average pay of 7.4M per year. At this point only his base salary of 7.4M has any bearing on his value relative to what he is getting paid. Obviously, he was a better value at 7.4M per when he was younger than he is now; however, the Cowboys don't have a replacement for him. Also, he is a team leader and savvy veteran to have in the huddle with a very young QB and RB. They do need to find his replacement, but until that happens, they need to keep him. Unfortunately, the only other skill position offensive player that you might call a savvy veteran is Beasley and as a slot WR that does not play all the snaps. Also, a 5-8 non-Pro Bowl type guy probably doesn't have a big influence on the huddle anyway.
Paying 7.4M for 1 year is not a big deal for a savvy veteran, team leader, especially when you have a very young QB and RB and none of the other skill position players are savvy veterans. If his salary was 13M like Dez, then that gets into the price range where you can't keep the player unless he is a top producer.