As to why they let those guys go, free agency was different back then. The Messersmith decision was fairly recent and many teams did not handle it well. Their were a lot of old time owners/gm's who had just recently been working under the old system with no free agency. Many teams seemed to take free agency personal and negotiations got out of hand, which is what happened with Lynn, Burleson & Fisk. Supposedly, the Red Sox front office forgot to send Fisk some sort of contract extension by the deadline and the commissioner declared him a free agent. Talk about stupidity. The Big Red Machine did the same thing in letting free agency break them apart. Many old school owners and gm's were not equipped for the new world of player movement and they screwed up.
I believe it was the book Game Six by Mark frost that I read about six months ago that discusses not only the 75 World Series, but the future seasons for both the Red sox and Reds, detailing how free agency blew apart both those teams as durantjerry said.
GM Bob Howsam put the Big Red Machine together. The Reds ownership had made a conscious decision NOT to play the "new free agent game" and, rather than oversee the dismantling of his team, Howsam stepped aside for successor Dick Wagner, who oversaw the dismantling of the Big Red Machine (losing Gullett, Rose, Morgan, Perez, etc).
Howsam was old-school, as durantjerry said, and would not have responded much differently to free agency than Wagner did, however. Reds were probably gonna do the same things regardless.
I think the Red Sox would've been different.
Dick O'Connell, who put the late 1960s-mid1970s Red Sox together, lost his job when his huge supporter, owner Tom Yawkey died in 1976. O'Connell, who wanted to negotiate with free agents like Lynn and Fisk, was fired by Jean Yawkey and replaced by Haywood Sullivan, who lost Fisk, Lynn, Burleson, Ferguson Jenkins, Bill Lee, etc.
Tom Yawkey loved O'Connell; Jean Yawkey did NOT.
Had O'Connell not been replaced, I think he would have done a much better job guiding the Red Sox into the waters of free agency. For example, O'Connell engineered a trade with the A's in 1976 that would have netted Rollie Fingers and Joe Rudi for $2 million, only to see the trade negated by Bowie Kuhn. O'Connell was much more proactive. I believe both Fisk and Lynn would have remained Red Sox had Jean Yawkey not kicked O'Connell to the curb and promoted Sullivan(Sullivan DID draft his son Marc Sullivan, a Red Sox and ATG great--we need a sarcasm emoticon). The Red Sox would have been much better off had O'Connell continued as GM, IMO