I think the initial shock is over for most. Many of us thought they'd pick an immediate contributor for a team that is built to win now. But when you listen to the interviews since the pick, you understand what went into the thought process. Its not something they just decided on the spur of the moment.
Could Simpson have been available a little later in the round? Its possible. But you gotta have the right trade partner to pull it off. And its also possible a team like the Jets or Browns who need a quarterback might pick him ahead of the Rams. Or possible trade ups from team early in the 2nd round. There were rumors the Cardinals were showing interest in that.
Was it a reach to pick him at 13? A position they don't need to fill right now? Probably. But if he's the guy they were targeting all along, they did not want to risk losing that.
Then there's the rookie contract aspect. If he turns out to be that guy, paying him on the rookie deal allows them to retain some of their key players who are gonna be free agents after the season. Dotson, Parkinson, Puka, Avila, Turner and Young after this season. Verse, Fiske and Kinchens after the following season. They are all important pieces to maintaining a high quality roster.