But that's my point - that the chance was taken by Michael. He gave Fredo an indefinite lease on life. Mama could have lived for 20 more years. I see that as a clear repudiation of your assertion that Fredo was a continuing threat. You don't seem to factor that into your analysis.
Michael probably knew his mother was already sick and her health was deteriorating rapidly, otherwise he wouldn't have made that decision.
And nobody has yet answered Olivant's question abuot what possible value Fredo in exile would have had to another Family.
And as an underboss and brother to Michael he should have known more secrets about the organisation which could ruin everyone who's involved.
