This sounds good, but I'm not really sure if it's true. Putting Francona to one side... most of the recent guys never managed again after getting fired (Kevin Kennedy, Butch Hobson, Joe Kerrigan, Ralph Houk, and — so far, praise god— Bobby Valentine). Grady Little, Jimy Williams and John MacNamara had short, poorly-received stints after the Sox. You have to go back to the 70s to find Sox managers who hung on for more than a season or two in their next jobs— Don Zimmer, Darrell Johnson. I can't really weigh in on these two— it was before my time— but my impression is that baseball was more of an old boys network in this era with a small circle of approved managerial candidates who rotated through near-infinite 'second chances'. Nobody demonstrated a clear track record of success after Boston other than Terry Francona.
I think it's fair to say they just haven't been very good managers as a group, and that this says more about poor decision-making from ownership and management than it does about media or fan perception.
I wonder if this track record is really any much different from that of other long time MLB teams, however. Each year, most teams (some would say all but 1) have disappointing endings to their respective seasons.
Sometimes managers are truly in no-win situations. Darrell Johnson was fired mid-season in 1976 after the team struggled out of the gate, a 6-15 start dooming their season. But it's unclear any manager could have done any better during the "hangover" season, especially with the team already bungling their first free agents (IIRC, Lynn, Fisk, and Burleson were basically unsigned until late that year, and expected to be traded at any time). Johnson then managed a Mariners team that was terrible even by expansion standards, as the franchise had absolutely no money to spend on players or scouting. Houk basically retired from baseball after his stint with the Sox ended; he was forced to manage the transition from the powerful teams of the late 1970's to the mid-80's Clemens era.
You are indeed correct that there was very much an "old boys" network in place during the 70's and 80's. McNamara truly epitomized the old boys network; he had 4 thoroughly mediocre (or worse) stints before being hired by the Sox in 1985. However, Darrell Johnson's Boston gig was his first managerial job after being a long time coach. Zimmer was on his second, but his first one with the Padres was very brief (and those Padre teams were just God-awful), and he was also a long time coach with the Sox prior to 1976.
The hiring of mangers is made tougher by the fact that projecting performance from past W/L record is almost impossible. Houk won 3 AL penants and 2 World Series with the Yankees (albeit 20 years before his hiring by the Sox). Meanwhile, Tito's record was pretty awful with the Phillies before being hired by the Sox. And there's the infamous Joe Torre example.
Yes, there truly were some awful hires: Kennedy, Hobson, Kerrigan and Valentine were truly the worse of the worst (although the all time title for the worst manager award is probably a split between Joe Cronin and Joe McCarthy). Fortunately, only one of them was hired by this ownership group, and that was under some truly bizarre circumstances (Lucchino was trying to do a favor to an old friend is my read of that situation, and was unfortunately allowed to get away with overruling his new GM on the matter). Whatever one thinks of Little or Farrell, their hirings were truly defensible under the circumstances. Little was hired towards the end of spring training when most candidates were already in place, had been bench coach of the team under Williams, and came highly recommended by Charlie Manuel. Farrell had a long history with the Boston pitchers and had been well regarded as pitching coach during his tenure here, and the team did actually have some success in his first year. None of the same could be said about the 4 members of the Hall of Shame mentioned above.