Your 2020-2025 stuff is incorrect IMO... You think in this situation where he'll be used a lot more, add tons of miles in GB, he'll be worth double in 4-5 years as opposed to signing elsewhere.
He is a top WR in GB and when he comes up for contract in another 4 years he will ONLY BE 28 years old with 3-5 good years still left in him likely. If he were to sign in Oakland or Jacksonville he would see his value take a major hit over the next 4 years likely due to his decrease in production.
I also think contracts will increase that much in the next 4 years that top WRs will be making around $13-15 mil/year. Him making $60 mil over 5 years is really not out of the question in my mind with the increase we likely will continue to see at the WR position he could be in position to make even more.
2011 WR Contract Average/year - 16.1, 11.4, 9.6, 9.3, 9.2, 9.2, 9, 8.9, 8.3, 8.2
2015 WR Contract Average/year - 16.2, 12.8, 12.8, 12, 11.2, 11.1, 11, 10.7, 10, 10
*Primarily a 2-3 mil/year increase over the last 4 years
Edit: Also about being used more? He would be use more but less effectively in Oakland and Jacksonville likely. Running him, screens, and targeted more would be what would happen in those two locations. The issue would be that he would be likely targeted more but with less success. Right now as Adams/R Rodgers progress and Abracadabra gets healthy Cobb has the potential to be targeted slightly less as Rodgers looks to spread the ball around.