Do I think the usage you described would be sensible for Krug? Yes.
But do I then think that the contract required to get him here would have been sensible for a player with that usage? No.
I don't know what kind of guy you're expecting to sign for just 8% of the cap then. His contract is pretty similar to guys like Ryan Ellis, Erik Johnson, and Tyler Myers. Guys who can go out, play 20-22 minutes, and have some pretty clear strengths and weaknesses.
I think that St. Louis basically dropped themselves from true contender status by swapping Pietrangelo, a true #1 all situations defenseman to Krug, a PP specialist. All to save what, a couple million dollars per year?
Can Justin Faulk be better? Maybe, and I think this is the biggest variable. I like the talent level but there's some games he makes Madison Bowey level mistakes.
I don't think Colton Parayko is as good as he's made out to be on these boards. Playing with Bouwmeester helped him considerably and they worked well as a pair. After Bouwmeester went down I wasn't too impressed with Parayko's overall play. It's like the Lidstrom effect to a lesser degree; anyone, even Dandeneault, looked good playing with him. Bouwmeester became a really steady guy defensively in Calgary and St. Louis and I think he elevated Parayko more than the other way around.
I wouldn't be surprised to see St. Louis being considerably worse defensively and finishing as a 5-8 team in the Western Conference for the next couple of years because of this move.
Here might be an unpopular opinion but maybe St. Louis was never really a consistent contender with or without Pietrangelo.
Tampa is really the only team in the league that I look at and see a consistent Cup contender, nearly regardless of what they still do this summer. Everyone else...yeah, playoff teams. If they make it in, anything can happen, because the league is pretty flat at the top. goalie gets hot, your star gets hot, and you can make a run. Someone goes cold...well, good luck with that.