Even if he regresses to let's say 25G and 60 points, which would be a huge regression given his role in Vegas and his line mates Smith and Marchessault, his defensive abilities more than establish him as a true 1st line Center. $6M would still be tremendous value at that point. Look at what Turris signed for and his numbers were far below what Karlsson should be projected at.
My issue with that thought is that you place his floor at 25G and 60 points. I think that is really optimistic, as I see the floor as much lower, and I would be surprised if he put up 25G or more this year.
The thing is, neither side wants to screw this up. Karlsson knows that even if he falls back to earth quite a bit, he can likely get over $5M on a longer term deal with the cap going up, so even if he could have gotten $6, why would he take it when he knows a big season would blow that out of the water? Most athletes would take that bet on themselves.
Vegas shouldn't be offering a nickel over say $6.25M on a deal with any amount of term. I think Karlsson is a good player, but that season was an aberration. He didn't generate enough chances to warrant that type of production, but pucks went in. There is no way you can pay him based on that production, and I would be very comfortable betting on his shooting percentage taking a massive dip, likely below half of what he did last season.
I think this is a case where the player will be on himself, and Vegas was not going to offer enough of an incentive for him to skip on that, so a one year deal just makes sense.
If something miraculous happened and he did put up over 30G on 180 shots again, I wouldn't want to sign him for that and would just say thank you, trade him, and let someone else overpay for him in terms of assets and salary. There really isn't that much risk for Vegas. Him putting up another massive season will make him a tremendous asset, and if they still don't feel it's sustainable, RFA's are easier to trade than guys with big contracts.
Personally, I think he will regress very heavily, and may struggle to score even 20 goals. If he misses the 20G threshold and puts up 40-50 points, he is still a good player, but the disappointment likely turns his salary into a great value on a longer term deal. If he has a good season, in the 20-25 goal, 50+ point realm, you are getting him for a number similar to what you were saying anyway, or maybe still less.