They've had the best record since that draft because they had elite players in Rask, Marchand, Bergeron, Krejci and Chara all on sweetheart long term deals, all of whom were acquired and whose deals were signed by Chiarelli, not Sweeney. Chiarelli drafted Pastrnak too. Literally the only good draft picks the Bruins have had in Sweeney's entire tenure are McAvoy at 14 overall (and he's been overrated and struggled since the main core has moved on), Carlo in the 2nd in 2015 which was a fine pick, and maybe one could argue Frederic, although I don't think he's all that good.
The Bruins had an entire elite stanley cup winning core locked up to sweetheart deals and a ton of cap space to build around them and be a dynasty, as good or better than the Blackhawks, Pens, Lightning. And they came up far short mainly because of how pathetic of a GM Sweeney is. Don't get me wrong, I love it, I hope they keep him there forever. But he is by far the reason they did not win another Cup with that core and also the reason their roster is in such terrible shape right now and going forward. And yet they keep firing coaches who go on to win elsewhere. Lol