Just doing some quick math here:
I can pay him $2 million for 2019.
Tag him at $28 million for 2020.
Tag him again at let's say $35 million for 2021.
So I would have him for the next three years for a total cost of $65 million. An average of less than $22 million a year.
Why the heck would I want to sign him to a long term contract paying him in excess of $30 million a year?
They'll get 2019 for 2M even with a new contract.
The Jaylon contract is:
Original Contract 2019
1st round tender amount 2020
New 5 year contract starting in 2021.
The 12.75M average announced was for the 5 years. The average would be lower if all years are considered.
The structure does not "look" exactly as described because they pay the signing bonus when the contract is signed; however, the numbers come out as described above.
That's how most contracts are done now when players have a year or years remaining at the time they sign the new contract.
The agents like being able to "advertise" the higher average and teams are not going to give away the remaining years of the low cost rookie contract just because the contract was signed early.
Generally teams only want to re-sign players early if they can get a more team friendly deal; otherwise they would all wait until the contracts have finished.
Side Note: It appears that in March teams can use the Franchise Tag on 1 player and a transition Tag on another because it's in the CBA as an option for the final year of the CBA agreement which is 2020.
It would still be beneficial to the team to Franchise for 2020 & 2021 at 28M and 35M for a 31.5M average but zero risk.
Dak would holdout if Franchised, especially the 2nd time but the threat of the Tag is how teams keep the agents for QBs from requesting 95% of the cap space instead of the 18% they're already requesting.