These are good reasons for why NFL teams value first rounders more and I essentially agree with you. For a number of reasons, the draft is more essential to NFL teams because they need more players. More specialized roles (you can swap perimeter players through roles or play a natural PF at center, but it's harder to swap a linebacker into a safety role, let alone a wide receiver role), more good players required to win, more units necessary for a variety of schemes.
My point was simply that draft pick bounties on free agent signings are a significantly more powerful disincentive to sign away another team's free agent in the NFL than the NBA. You'll never see an NFL team sell a first round pick for cash (if such a thing is even allowed by rules...but even if it were, no team would consider doing it).
