How about offsetting the loss with these supposed substantial gains later on?
Also, we don't know which side would absorb what percentage of any loss. If it were a dollar for dollar removal from the cap, the players would be eating all of it.
If this gets to where Dallas cant afford its quarterback (first of all you'll see other players cut first, and very possibly Prescott getting nervous and caving), many, many players on other teams are going to need to be cut and be on the sidelines. The league doesnt want that, and wont force the NFLPU into it.
This supposed loss, which is pure speculation at this point, will be barely noticed as far as player cuts go.
Some people just want Prescott out of town so badly that they spend time coming up with scenarios under which the NFL owners will collectively shoot themselves in the foot and lower the value of their franchises.
Sorry, not gonna happen.