But why didn't WB oneshot the admirals, then? Sick or not, he attacked Akainu with full killing intent, adrenaline and bloodlust and got some clean hits in with his DF on top of his haki.
Akainu was healthy enough after taking those to still absolutely dominate the likes of Jinbei and the whole WB crew.
Marco and Jozu's respective durabilities are really special cases. Especially Marco, since he just instantly heals every hit he takes, one cannot accumulate damage on him through normal means, and unlike logia, being attacked with haki doesn't counter his DF.
Of course the Marines had an advantage on Marineford, there's no denying that, but would the results really differ extremely? Let's think about it.
I'll start by ignoring the "healthy" WB premise, simply because "healthy WB" is not a thing. He aged poorly and he was constantly ill. We have zero information on how strong he was when he was young, but we know that he was still 100% a Yonko when he was sick, so it's fair to consider this form of him here. Otherwise I could also say "well, Garp was old" and go by the hype of his prime power.
Why do you want to take the Warlords away but still leave the allied pirate crews that are not part of WB crew (there were multiple pirate captains helping him on Marineford)? If we take away the external help, then there would also be many less pirates. Not only that but, as others pointed out, Warlords ended up being more of a help to WB than they were to the marines (1 switched sides, 2 attacked both sides, 2 didn't care, 1 was useless, so pretty much only Kuma was really an asset for the Marines) so taking them away is more of a nerf for the pirates side.
Other great fighters like Crocodile, Ivankov, Inazuma and Oars Jr. were not part of WB's crew so they would also not be there if this was just "plain fighting" between WBs crew and the marines. Lucky for Whitebeard, Squardo would also not be there. If Squardo was there, he would still betray Whitebeard in this new setting, because Akainu would still manipulate him behind the scenes.
The only big advantage from this new setting is not that Ace's free, but the change of location. In this new location, Sengoku can't lay traps so he can't shine as much as the strategist that he is, but of course he would still have at least a plan and all of the Pacifistas that they used to secretly surround the pirates would also be brought here together with the human marine troops.
What would really, really change in this setting?
This would still be decided by the top tiers in the end. Whitebeard can obliterate most if not all of the marine fodders just like Akainu can do the same for most of WB's crew.
WB, Marco, Jozu and Vista (4 people) are the only WB pirates that are worth mentioning in the matchup, and Sengoku, Garp, Akainu, Aokiji and Kizaru (5 people) are the only marines worth mentioning. I ignore Tsuru because we don't know anything about her actual battle potential. Sure, she is considered very strong and she started as the teammate of Garp and Sengoku, but for all we know she could be their Sakura. Ace is strong and had huge potential, sure, but he was oneshottable by Akainu so I don't even take him into account here. WB, however, cannot oneshot any of the marines I mentioned.
Did I forget anyone? Let's compare these guys.
- In the original MF fight, Garp let himself be punched out of the way by a mere G2 Luffy and was severely enraged by his fellow marine Akainu because he executed Ace. Shall we ignore Garp's dubvious loyalties and morals to make this "plain fighting" more fair? Then I can say for sure that at least Garp > Jozu, and it's also very likely that Garp > Marco.
Even if one cannot prove that Garp can "break" Jozu since we haven't seen the limits of Jozu's durability, Garp should be strong enough to punch him out of the island. Not in one shot, since Jozu also showed to be plenty strong and resilient, but Garp is enough to have him out of the game.
- Let's say that Kizaru is enough to either beat Marco or at least keep him in an eternal stalemate (if there's not "distraction" plotline), so let's say that they can both distract each other long enough from the main fight.
- Sengoku and Vista and pretty much wildcards here. Is Sengoku's physical strength comparable to Garp's since he was able of holding him down without using his fruit? One thing that's true is that Sengoku's buddah form took on all of the BB pirates together at the end of Marineford, so I'll say that he can take on Vista. We don't know the exact limits or abilities of any of the two characters, so who would you say that would win between the two? None of the two can oneshot the other, and none have shown what it takes to hurt them, so I will just say it's another stalemate (for lack of feats).
- WB ragdolled Akainu on Marineford, yes, but Akainu did not go down and as I mentioned earlier he still was healthy enough after all of that to still be a monster that the whole WB crew, Vista and Jozu included, considered they had no chance of beating him in that state. And while Akainu took those heavy hits from WB, WB got a hole in his stomach and lost half of his face, so it was not one-sided at all. If we take away Squardo's treason and plot, I'll take away WB's bloodlust from seeing Ace die before his eyes. I'll consider that WB engages Akainu while Ace is busy fighting elsewhere. WB would be a bit healthier but he would be less of a monster against Akainu.
And there is still one toptier, Aokiji in the examples I went for, that is left unengaged by any proper WB subordinate. By Jozu? Sure, then who blocks Garp? Ace didn't live enough to be on Garp's level and, wihle Marco may heal from any damage Garp deals on him, Garp can just swat him away while he goes engage WB.
The war would end up with, at least, a 2 vs 1, if not a 3 vs 1, top tiers vs Whitebeard, and while the commanders were strong enough to delay some of the admirals, they were constantly on inferiority and their defeat was a matter of time.
WB would lose is the likely result, and this is without considering that probably Akainu is able to kill him in a 1vs1. I consider that a fight between them can go either way, similar to Akainu vs Aokiji, but in this context, it wouldn't result in a 1vs1, so WB is screwed.