Carlo is not Rantanen, a top ten player in the league and game breaker type talent. The Oil weren't going to mortgage their entire future to push for a second d-man, especially a right shot guy within very competitive market conditions. Unlikely they could match the Maple Leafs who have some good pedigree prospect overlaps to deal. This is a huge playoff and the Oilers organization may face some bigger roster reshaping if they can't replicate the deep run. Will need some assets to ensure flexibility of future needs.
It's a nice thought. I believe in having deep, versatile and ideally long d-corps for hard, playoff grind series hockey. Especially to help fortify volatile goaltending. But don't think the Oil would have won an arms race for Carlo and this roster post Summer of Jeff was left with holes in all roster areas. Oil made the best d-bet for their primary need with the two way puck mover with utility to play in all game situations and flexibility to move between LD and RD.