Utah is dealing for Larkin because their time is now. Cooley is 22, but look at the ages for all of the other movers and shakers in that lineup - they are 25-30. For as good as Desnoyer and Iginla could be, they're both likely at least three years out from being real contributors. Utah can wait on that, but go thru their lineup and add 2-3 years onto everyone. Some of those guys, especially their blueline and goalie, start looking pretty grey haired by NHL standards.
I think Utah looks at this and sees Larkin giving them that legit 1-2 punch down the middle right now. I would bet they are also looking at Alex Tuch. Noise has been that he's likely looking to move west to be closer to family. Vancouver is awful, Seattle isn't much better, but Utah is in a good position to offer him a nice paycheck and the opportunity to win. SJ and Anaheim can make a similar pitch, but Utah with Larkin looks further along to me. And Tuch might just not like Cali taxes.
Also, I think the western conference is wide open right now. All of the top teams have issues, whether it be positional depth, age, inexperience, etc. Good time to try to jump in and be a big player, imo.