Right now I'm glad he forced us into a shorter deal, and while I think he can be salvaged I wouldn't be against having a contingency plan at QB.
I don't think there is a contingency plan unless we're willing to take a QB in the 1st, which would more or less necessitate us getting rid of Dak ASAP.
Look around the league, what team has an elite-level QB taken in the mid-rounds?
Cousins?
Feh.
Wilson?
Maybe, but what has he done since their all-time caliber defense fell apart?
About as much as Dak has done the last 5 years.
Unless Dak declines even further there's really no other option except continue building up the defense and hope you can make a run without asking him to do too much.
But if he plays as he is now over the next 3 years he has to have some self-awareness and be fine with a deal that is only ~85-90% of the top of the market. If not, then maybe you try to trade him ahead of time and hope you have a strong defense that can ease the transition for a rookie QB.