Why not? Kupp isn't a pig-in-a-poke gamble anymore. I would pick him in a heartbeat in the 1st round if I knew what he would become. We are not talking about an Austin Pettis here, but one of two sure handed receivers who are almost gtd to come up with a critical catch when it matters. The best Rams duo since Torry and Isaac.
Again, Kupp has been an outstanding slot WR. But, the attention defenses have had give to the likes of Woods, Cooks, and even Watkins on the outside has played a role in his success. Most NFL defenses simply don't have 3 quality CBs and Kupp has benefited from it.
I wouldn't take an outstanding slot WR in the first couple of rounds for the same reason some wouldn't take a RB in the first round. The position is easier to fill (much easier than RB, IMO). That person may not be as good as Kupp, but under the same circumstances, he wouldn't have to be as good for the team to still have a lot of success. If we could go back in time, I believe that a healthy Danny Amendola could be just as successful under these circumstances.