The key is, assuming that Dak continues to play at a high level, we would HAVE to get the deal done after the 2019 season. After 2018, Drew Brees *may* exceed Stafford's deal, he may decide to save the Saints some money to improve else where at this point in his career, but we'll see. Kirk Cousins could as well, because like Stafford, he hasn't won a playoff game, but I see him coming in a tick or so below, if he hits the market. Winston and Marriotta are two big names that will be free agents, but their teams have the 5th year option to play with. Matt Ryan, is the biggest and will exceed Stafford's deal, just a matter of how much and Dak's, Winston and Mariotta's agents, likely would not extend their client until after Ryan signs a deal.
But in 2020, Aaron Rogers, Russell Wilson and Ben Rothlisberger will all be free agents. Rodgers and Wilson will exceed what ever Ryan signs in 2019, and that is likely to push the value of the top QB to $30 million or beyond. Assuming he's an elite QB, then Dak's third deal is when he should set the market, just too much leverage to not use.