If Dallas doesn't win a championship in the next couple of years the team will explore other options at QB.
If Dallas finds that option, it won't matter how many years are left on the deal. Prescott's contract will not protect him. Dallas will take the hit and move on.
During Jerry Jones' stranglehold on general managership, 'next couple of years' has been relative and 'exploring' would be unique. Troy Aikman helped lead the team to a Super Bowl XXX victory. Couple of years = six years when he retired from suffering too many concussions. Jones explored and found Quincy Carter only after Aikman retirement.
Tony Romo led teams never won a championship during his ten season stretch (non consecutive due to injuries). Jones seriously explored neither Stephen McGhee nor Dak Prescott as his successor. Romo's preseason injury provide Prescott with his opportunity to start and Jones put his full support behind his new franchise quarterback midway through the 2016 season. Couple of years = circumstance.
It is currently the Dak Prescott era. Seven seasons. Jones has made zero serious draft selections as Prescott successor. The hypothetical length of Prescott's next extension is way up in the air. Just not sure, based on Jones' history, that a 'couple of years' with the team not winning a championship actually translates as a 'couple of years'. However, circumstances, similar to what happened to Aikman or Romo, might either prompt Jones into being proactive or be graced with luck. A couple of years and/or exploring other options might be taken out of Jones' hands again if history repeated itself.