THe Buccaneers added Antonio Brown, Gronkowski, and Fornette to win the Superbowl.
Adding CMC, and OBJ would be no different.
Trading Singletary for CMC isn't as big a deal because Devin will be gone next year. It's the draft pick that will hurt more.
Also, the Bills are not going to bench McKenzie, or Knox to give Singletary more snaps.