First of all, I know the difference between objective and subjective. It seems you don't and heres why.
The most important thing you seem to forget is that MMA rankings themselves are not objective.
You make the statement that objectively speaking, Brock Lesnar and Bigfoot are equal wins to Overeem and JDS. That is false. The only thing objective about it is that they were all ranked in the top 5 at the time of each win.
To put it simply, these are NOT objectively indentical accomplishments. The objective argument actually FAVORS Stipe because he has more consecutive defenses whereas Cain has a loss in between these wins and thus, is an objectively a less accomplished champion that Stipe. These are the objective takes.
The subjective takes would be the quality of each individual win. And a ranking does not objectively dictate the quality of a win because, again, the rankings themselves are not objective.