This is a perfect example of someone applying their subjective opinion rather than looking at tangible evidence.
You have no way to prove another manager would do worse but I can post statistical evidence that Ole is infact the worse manager we've had post Fergie. This is what ultimately this debate boils down to and because of Ole's history with the club it has created a sub section of supporters than seem incapable or analysing the seaosn objectively.
Except that football is not devoid of context. Specially when comparing different years for the same team. You may as well compare the points totals of Leeds and Bournemouth and call it "objective". It's only objective if all parameters other than the one being assessed are similar. And it's very different for Ole vs LvG vs Jose.
LvG - took over a team that finished 7th under Moyes but had been champions just the season before. It still had multiple world-class players in it. He inherited RvP and Rooney in the strike force for example. He was also given oodles of cash to splash and was in a league that was not so competitive.
Jose - took over a team in worse condition (to LvG's first season, not Ole's), but which still had not completely lost it. Just won the FA Cup and he too was given a blank cheque really. Atmosphere was optimistic about building on this - and there was no sense of turmoil and despair
Ole - took over club that was now clearly in a horrific downward spiral and a dressing room as well as overall structure that was borderline depressing. Also took over a squad with an unimaginable number of players that were just dross. Think about it. Since he took over, we've sold / released (not counting Herrera) Young, Valencia, Lukaku, Fellaini, Darmian; loaned out Smalling, Alexis, Rojo and yet we still have on our books, Jones, Lingard, Mata, Pereira, Dalot.
It is absolutely (in my opinion) not correct to simply look at the points and say it's objective.
On transfers, Ole has been the best; in terms of improvements in individual players, none delivered like he has; in terms of playing style, none of the others produced any spell or run like the one we're on and on general atmosphere, nobody built this. Are these subjective? Sure! Apart from transfers, they are. But comparing managers just isn't amenable to simplistic and reductive "objective" assessments - at least not in my opinion.
PS: When you say Ole is the worst post-SAF, technically you're even mentioning Moyes. The David Moyes who took us from 1st to 7th in a single year. Yet, by your definition of "objective", Moyes is currently a better manager? Surely that alone should make you pause and rethink whether you're on the right track!