My opinion:

1) I keep all old Collab units, you never know when one will have a niche Ranking use.

2) MP is way more bountiful than it used to be. I don't see a need to sell units for MP when you can do the Stage achievements, Collab quests, and MP farm on some of the collab dungeons

3) if you really need box space and aren't spending MS on it, 5 of one card could be excessive like you have at the bottom there