And if you stick to the BPA strategy, you can end up like the Cowboys who are now overloaded at WR with Cooper, Lamb, and Gallup all acquired with good draft picks three years in a row. They have three damn good WRs at the cost of being weaker elsewhere. That's why the whole best player available thing can really come back to bite you.
Given what was paid both in terms of the pick and the cap space, it was going to be very hard for Cooper to be worth it. Trading a first rounder for a guy wanting to be one of the highest paid at his position causes double pain, especially if the guy turns out to be very good but not superstar special.