Why would a team ever be stupid enough to "assure" a player that he will be the pick if available at their spot? If rumors are true (or is it an actual quote?) that Arians told Mahomes he'd be the Cards' pick, that's the reason the Chiefs traded ahead of the Cards to get him.
If this rumor is true, it just gives teams an opportunity to do the same this year, plus lets QB needy teams know that they don't need to bother making an offer to the Cards, they can just deal with the Chargers. For example, the rumor about a three-way trade with the Chargers and Vikings where the Cards still get Harrison at #5 while moving from 27 to 23 is off the table. The Vikings can save a little by dealing directly with the Chargers.
@Harry, do teams really tell players they'll be the pick, and if so, what possible benefit is there for the team OR the player? If this is just another way of putting up a smoke screen, it's a pretty lousy way to use a player as a pawn in the game.