Kuper doesn't make that much by starting OL standards. He's making less than half the franchise tag, so pretty average for a long time starter.
He's also the elder statesman of the OL at just past 30.
When he was healthy the OL was clearly far better, and most importantly Franklin played his best football having an experienced vet next to him.
Spending draft picks to replace a guy of his caliber and at a relatively young age for the position is much more expensive in terms of opportunity cost than the $4.5M he'll make next season. Drafting a young interior OL guy to groom and challenge Beadles, Walton, and Kuper long term? Sure, in the middle rounds. But the early picks need to be focused on defense and giving Manning more weapons (RB and WR), not backfilling a hole we created to save a few sheckles.