The overarching point is if you traded up for say, Lamb. (IMO the best all around WR in this draft), the best you could reasonable hope for is that he turns out to be as good of a player as Diggs. But there’s no guarantee he would have been. And you’d have to trade more capital to secure it. Diggs is the bird in the hand. Out of the top 3 guys, maybe one of them will turn out to be better than Diggs. But there’s also a significant chance none of them will be better than him at all.
Bird in the hand. Known commodity. Easy decision.