Boston fans never even heard the phrase "sustained" contenders in the final decades of last century, since it was a given the Sox already were... probably because the Red Sox continually developed star players, and almost always found a way to keep the best ones.
Someone noted recently about a short span in the 1970s when the Red Sox traded away Reggie Smith, Cecil Cooper, Ben Oglivie and Juan Beniquez -- all became either home run champs, RBI champs, .300 hitters, Gold Glovers or All-Stars -- and Boston fans hardly missed any of them, except Cooper, who got serious AL MVP votes for five straight years (while the guy he was swapped for bounced into a million double plays).
The lack of regret for losing good players is that the Red Sox always had plenty of worthy replacements -- Brady Anderson was expendable because they had Ellis Burks -- and management was always ready to supplement the roster with any stars that became available from other clubs.
The Yankees were also always on the prowl for upgrades; still are, but now they can have more of their picks, with their old archrivals sitting this one out.