Hindsight is what it is, but I've even saying this since the big 3 (Stafford/Suh/Megatron) were still together.
The biggest regret the Lions may end up having is not trading Calvin for picks before 2015. During that stretch in 2014, Stafford showed that he played more productively without him on the field, and if we'd had a chance to keep that pass-rush together, who knows where this team would be now.
Now you've got Stafford playing at a level that could get him into the MVP conversation if the Lions were to somehow steal the division, but nothing to show for Suh & Calvin walking.