Honestly, I never subscribed to this theory that you should spend all your money on the top 4 / top6 and the rest of the team plays for league minimum. I don't think it's realistic. I think you need a $3m 3rd pair D to anchor that pair and be someone who can slide into the top4 for injuries. I think you need one pricey 3rd liner for the same reason. I think most teams spend money on 7 F's and 5 D. (I wonder if that's why the expansion draft chose those numbers.)
I also think the Bruins have a healthy cap allocation. I don't see any bad contracts or money wasted (I'm giving Lindholm and Zadorov an Incomplete because it's only 20 games). Even Coyle, who people love to jump all over, put up 44 even strength points last year, and 40 the year before that. Most 2nd liners average 35 even strength points a season. Bergeron, his last 4 years here put up less than 40 ES points. Marchand put up 37 in each of the previous two seasons. Mikael Granlund, the big prize rental at the deadline this year, average 36 ES points. Chris Kreider, a guy the Bruins have long coveted, averages 37. So I think the Bruins have gotten their money's worth out of Coyle so far.
Long story short, I don't think Korpisalo was a bad acquisition from a cap perspective. If they didn't get Korpisalo in that trade, they would have signed a UFA to similar money for the same role. I do think Korpisalo had negative value at the time of the trade, so from a trade value perspective I think it's fair to be critical.