I agree that there's a vast difference between Bob Kennedy and a runner who has been performing at 13:08 or 13:10 for a few years.
Prefontaine lowered his 5k personal best by one second between 1972 and 1974. In 1976, the bronze medalist had a PR of 13:13 going into the Olympics. (Maybe that's why Rod Dixon finished fourth.)
My point is that Prefontaine would have had to make a quantum leap just to have a chance at a medal in the 5,000 in Montreal. Going from 13:21.8 at the age of 23 to 13:12 by age 25 would have been quite a feat.
In any case, the guys with faster PRs than Pre also had stronger kicks. That's why I never bought into the myth about Pre being able to win gold in Montreal at the 5,000. At best, he would have been 5th behind Dixon.