So the uptopian goal of your starting rotation would be to have every single pitcher throw a complete game shutout/ CG quality start in every single outing. (If you had one guy with superhuman durability then you'd only need a 1-man rotation, but it's not 1890 so we know that goal isn't possible.) Generally the best order of a rotation is to have them go in descending order of ability (re: statistical effectiveness).
Since it is obviously not realistic for SP pitchers to pitch 100% of innings based on stamina and ineffectiveness, let's assume that your staff ace could go 9 every game. This gives your entire bullpen a day of rest. Then, the next best case scenario would be to have your #2 starter pitch 8 quality innings and use your next best (relief) pitcher as a 1-inning closer (when the game is close. If the game is not close, whether winning or losing, you can have anybody- your worst guy, whoever- come in for 3 outs because those outs are meaningless outs). The next best case scenario would be to have your 3rd starter pitch 7 quality innings, have your 2nd-best reliever pitch a quality 8th inning, and have your best reliever pitch a quality 9th inning. If your 4th-starter can only go 6 quality, have your third best reliever pitch the 7th, your 2nd best pitch the 8th, your best pitch the 9th.
I'm sure you are noticing the trend here. In this scenario, your starters go as far as possible and then turn it over to bullpen. Your closer (ALWAYS YOUR BEST RELIEVER) makes the most appearances and makes them in as crucial of a situation as possible. Your 2nd-best reliever (set-up-A) makes the 2nd most appearances and makes them in the 2nd-most crucial situations as possible. And etc etc.
If you are asking me why my way is right, the answer is, "Math." If you are asking me why your answer is wrong, the answer is also, "Math." I'm explaining the concepts to you in words and not numbers.
"In pj's perfect world, in which blackjack game theory can be applied to baseball, Mariano Rivera's best season would be one in which he sits in the 'pen the entire season and never has to make an appearance." Correct. In the utopian world, their new Japanese guy would be superhuman and pitch 2000 innings and go 162-0. That goal is always the goal regardless of its attainability. That math is always the right math whether it's realistic or not.
I don't know why this is making all of your heads explode. Maybe y'all should stop trolling with your one-line gang-ups and actually think about it.