It doesn't really matter how much fans complain, what matters to team owners is how they vote with their feet and wallets, i.e. how many still turn up to games, and how much merch do they buy? As long as those figures remain high, then all the rest is just noise. As far as I know, Pats attendance hasn't dropped off, whereas Fenway's average crowd numbers have decreased but not by a massive amount. As such I would argue that there is a certain level of tolerance amongst Boston sports fans for teams to go through rough patches. Social media and radio amplify the angry folks, but the more sensible middle seems to hold up better.
How long that would remain the case, and whether the same would be true for the Bruins, are different questions. What I would say is the Jacobs' shouldn't dismiss the franchise's ability to wear a rebuilding phase out of hand. There's at least some evidence that a solid percentage of the fans will stick around.