The team currently has a projected 93m in cap space next year, 4th in the league. Even with Watt signing likely to take up a good chunk of that, the chances of them being in a position to sign someone who'd wipe any comp valuation for Pickens right off the board is pretty good.
I mean it's 90% sure that 90% of the reason he's gone is that he was too much of a crashout for Tomlin to want to continue holding. But in terms of pure asset management, getting the guaranteed pick rather than the maybe pick - and getting it about 10 picks ahead and a year early - seems a pretty reasonable return for moving off of a WR a year early.