$35m in 2019 = $45m in 2021, is the point.
Let's say the cap grows $13m to $213m for the 2021 season (same growth as the last three years). Before this season, $35m AAV represented 18.6% of the cap. The same rate on a $213m cap is $39.6m. So you've effectively gotten to $40m just from inflation. Add into the equation lost dollars from playing on the tag and rookie deal, and growth as a player and you're at $45 easy. Add your Super Bowl escalator, and you're near $50. There are 500 factors - guarantees, bonus structure, term, etc. - that will influence AAV, but the price on Dak is only going to go up. Even if he regresses a bit next year, you're still looking at nearly a $35m quarterback, because that's what they cost.
There's too much sticker shock with salaries, and there's a reason Dak didn't want term this offseason. The cap could explode in the next CBA, and $30m would be low. Dak is smart to not take much term beyond 2021 for that reason, and fans need to understand that what seems high today might be low in 2 years.