Not sure I agree.....
Coop is will make $13.9M this season. 2019 franchise for WRs is $16.7M. OBJ is the highest paid WRs at $18M followed by AB and about 7 players making $15 to high $16M. As you can see the difference between the franchise tag and highest paid WR (OBJ) is just under $2M. So this position has probably hit its salary ceiling range for next 2-3 years at $16-18M. Since Coop is already on the books at $14M ( 13.9 ) it'll only cost us about $3M to tag him for 2020.
Bottom line......if we are looking ahead at Coop contract, all we have do is save 2020 tag and budget approx an additional $3m and he's locked down for the next 2 years. Even when time to sign him 2021, $17M range would make him the 2nd highest paid WR behind OBJ's $18M.
https://touchdownwire.usatoday.com/2019/03/01/nfl-2019-franchise-tag-numbers-transition-tag/
https://www.spotrac.com/nfl/rankings/average/wide-receiver/
But Dak's numbers almost certainly going one direction with no ceiling the longer we wait ......^^^^^^.