i go on about this fight so much it even bores me but nigel benn v gerald mclellan still stands ouit for me as the most brutal fight i've seen. and benn was a huge underdog.
I am not discrediting the performance, hell it would probably be in my top ten, but duran got into leonard's head - he insulted him and leonard took it personally so he basically went out there and tried to take duran out, which is the wrong tactic (unless your hearns) to do against duran.
its like the douglas tyson one, the myth that douglas was 60-1 is just bollocks. you don't get 60-1 in a two horse race.
I'm sure men with the boxing brains of Sugar Ray Leonard and Angelo Dundee would have realized after a few rounds of Duran outbrawling Leonard, that "taking him out" was simply not possible. If Leonard could have changed the fight that night, he would have done. But he couldn't. That was because of the performance of Roberto Duran.
Not 100%, the source is Dark Trade by Donald McRae. It doesn't specify what the specific bet was - just "it was possible to lay money on Benn at 40-1".
It is highly plausible that they felt leonard could have taken duran out in the later rounds, with duran moving up in weight, they may have felt that leonard had more power and speed combined than anyone duran had faced before. Look at margarito vs Cotto - cotto was outboxing margarito but margarito kept with the same gameplan all night (not that he has any other ) and it eventually paid off.
It is highly implausible that if victory in that fight was as straightforward as "if Leonard had boxed he would've won" as you suggested, that Leonard would have taken the risk and the punishment of brawling for 10-15 rounds for the mere hopeful chance of a stoppage win.