Only problem with that logic is once Geb brought the record down to 12:39.36, it was not reasonable to expect someone else to better that improvement by dropping 20 seconds from his record. That would mean running 12:19!
I think a better gauge is world record duration. Geb was trading the 5,000 record back and forth with others, which shows he was not peerless. Bekele’s records lasted much longer, and he would still have those records if it wasn’t for super shoes and pacing lights. Plus Bekele had the cross country dominance that Geb never had.