O-Line play has a big impact on ESPN's QBR from my research and study.
It's not really worth using as a metric. Passer Rating is flawed as well but at least we know the formula, so we can account for its flaws. With the rule changes making it hard to compare ratings between older QB's and new QB's especially. Marino, Montana, and Young in today's game would be something else.
Dak piled up his numbers last year against the Redskins and the Giants. And was something like 0-8 against winning teams.
He is a good QB that has gotten a little better each year, but he is not transcendent. So making him the highest paid player would not be wise.
The problem though is QBs like Derek Carr have been made the highest paid, so the way it has gone the last 10 years is the next merely good QB up for a new contract becomes the highest paid. So Dak is justified in his negotiation.
If I were the Cowboys, I would remove the franchise tag and let him out onto the open market. I don't see a team out there that would be willing to give him anywhere close to what he is looking for. It's why Cam still remains unsigned as well, as all 32 teams are already decided on their guy at QB for the 2020 season.
The Cowboys have all the leverage currently.
This is a very interesting negotiation as it could begin a new precedent that teams are going to start wising up when paying QBs that are not future HoFers. Like the the Flacco's and the Carr's.
Dak's best bet is to go the Kirk Cousins route. Play on the Franchise Tag, and continue to bet on himself.
I hope the Cowboys break the bank for him btw.