No he should not....nor should any player. You signed a contract. You agreed to play your butt off and help the team succeed. Now, once that contract is up, you can negotiate for a better deal. Also, during your contract, if asked to rework your deal, you can put in stipulations that benefit you later.
Dak will get his money in a few years.
Imo, the cap is too rigid. Teams should be able to release players if they have been convicted of felonies, ate repeat offenders if domestic violence, and if they have failed a certain number of illegal substance tests (for illegal drugs, not performance enhancers).....without penalty. Also, no other team can sign that player for at least 1 calendar year.
Also, one cap amendment I would like to make.....since QB is the highest paid player on the team, and it tends to cripple the team with signing other positions....I would like the QB salary to not count against the cap.....but only if that salary does not surpass a certain % of the entires cap, say 15-20% per season.