The more and more I've thought about this the more it makes sense.
1: Diggs is 31 and prob only has a couple years left as #1. This draft is loaded and if you can get one of the top 3-4 WRs, you've effectively replaced him with a new #1 for the next 10 yrs. Trading him now, at least they got value to make the move to get one of those guys.
2: The trade clears his contract from the team going forward and gives them breathing room next year to move on from Miller if he doesn't produce this year.
3: dead cap is $31M. Diggs hit was $27M. So that's a $4M difference. Add on the rookie #1 WR cap hit, prob $2-4M and now that #1 cap hit is around $35M this year instead of $27M. They essentially are paying a slight premium to move on.
4: A rookie gives them, hopefully, a star WR on a rookie contract for the next 4 years or so, again, allowing money to be spent elsewhere.