First, I believe Diggs never played one down for his extension but he got a big bonus for it. We can thank Hill's contract in Miami for most of that 31M Dead cap hit this year.
Second, I know I am in the minority here but elite WR's are over valued IMO. What AFC teams have the best WR's? Houston, Jets, Dolphins Bengals...three of those teams are not in the playoffs and the one that is had a ton of injuries to WR's, Houston lost Dell, Diggs and Collins (for some of the season). The Chiefs traded one of the most unique talents at WR and then won the next two SB's with trash at WR. After QB, I really don't think elite WR is next in the importance list...it is probably coaching and scheme and players who can do what is asked at a competent level.