Agreed, setting PR's in the mid-late 30's seems inconceivable for someone who was at peak training during their 20's.
Regarding MJ, and disregarding everything VenTard says, there is no way he could have won the 100 in Atlanta as he was. Absurd.
However, if he had focused on the 100/200 to the exclusion of the 400, he would almost automatically have been a low-mid 9.9x guy.
Just at the start alone, he left .10 on the table, maybe .15 In the 200, his start was likely optimized to reduce impact, be as smooth as possible, and save energy for the straight.
If his body held up, I think that he could absolutely have been a mid-9.8x 100m runner, but he would have needed much more extension, and obviously increase his speed envelope, at some sacrifice of his excellent speed endurance.
Nobody has ever gone below 9.85 using anything close to the abbreviated technique of MJ. Look at how sub-9.85 guys run--they all have GREAT absolute top speed, especially the sub-9.80 guys.
The guys who were right around 9.85--guys like Burrell, Boldon, etc.--didn't necessarily have the greatest top speed, but had really good speed endurance, and that is the fastest that speed endurance will get you in the 100...the race is just so short that you cannot concentrate on speed endurance to the exclusion of other critical parts of the race.
Around 9.85 can be done with a decent start, decent top speed, and excellent speed endurance.
9.80-9.85 can be done with a decent start, really good top speed, and really good speed endurance.
Sub-9.80 can only be done with a great start, great top speed, and great speed endurance.
MJ never exhibited the first 2 of those 3.
IMHO it is doubtful that he could have gotten the second, ever, for whatever reason (inability, injury)--which is probably why he chose to run the 200/400 instead of the 100/200.