I appreciate the historic references, the departures of Lynn and Fisk were painful, shameful episodes by the real 20th century "curse" on this franchise, its dysfunctional management. But by the numbers they weren't as impactful as Mookie has been. Fisk's HOF career saw him rack up 68 bWAR in 24 seasons. I'm sure his impact as a franchise cornerstone would have been immense, so maybe you can equate the two in terms of what it meant/would mean to lose them. But even in that case he was 32 when he left and half his career bWAR happened in Boston. Lynn had two Betts-like seasons in Boston, 50 career bWAR, but never more than 4.7 for the Angels, whom he joined at age 29.
Betts has 42 career bWAR at age 26. For the Sox, only Williams was better in his first five years. Even Ruth started slower (but then of course went on to be the best player ever, probably) (but context... anyway). Yaz became Yaz starting at age 27. Clemens and Pedro, being pitchers, came up more slowly. So by the bWAR metric, for what it's worth, and looking strictly in a vacuum, losing Betts would arguably be the worst thing that has happened to the team since Ruth getting sold. Salary limitations and free agency are among the differences from the past century, and maybe they use the money to replace his contributions in a way the 1920 Red Sox couldn't (and the 1981 Sox were too stupid to pull off). But losing Betts would be a tragedy.