First, I agree that I won't be happy if the Lions are done upgrading the WR position this offseason. I expect them to take a WR somewhere in rounds 1-3, probably one of the guys you've mentioned, like Moncrief, Matthews, etc.
But what player do you think Detroit would've been able to sign with the cap space they had? And please don't make this an argument about "how dumb" Mayhew is for having a small amount of cap space. That's another argument. What free agent WR would've made the Lions not have a WR need anymore? This offseason the Lions needed to get a #2 and #3 WR, so even if they signed Nicks or Britt in free agency, they still were going to have a need at WR going into the draft.
Lastly, what defensive free agent could the Lions have signed that was a better option? Definitely not Peppers with the 7.5 million guaranteed that he got. And just because a guy like Alterraun Verner signed for about 6 million a year doesn't mean we could've signed him for the same amount. It was obvious that he wanted to play for Lovie Smith and he could've gotten more if he went somewhere else. Maybe we could've signed Chris Clemons (the safety) instead of Pettigrew, but no one knows yet how much Clemons is asking for or if he's a great fit for the Lions.