Being sick has nothing to do with not being the strongest.
Also, if you actually read Marineford war and actually read the SBS:
Whitebeard ripped his medical equipment prior to Marineford war, in fact.. he did it a day before the war which is why his condition was deteriorating quickly.
He ripped it off because he didn't need to be in his best condition to sink Marineford and die with the Navy. His goal wasn't to win and leave. His goalw as simply to save Ace and die with dignity knowing full well that even at his worst, he could come out victorious even with death and inability to fight to his full potential.
Even Marco points this out.
So, it's clear you don't understand the context of deterioration regarding Whitebeard and his title as the strongest. Whitebeard was the strongest.
Not true.
Can't become Pirate King without being the strongest.
Can't find Raftel without being the strongest.
There's a reason Roger and Whitebeard were the strongest in their era.
There's a reason Rayleigh specifically asked Luffy if he was strong enough when he said he wanted to be Pirate King (find One Piece) at Sabaody.
It's impossible.
If you are following the series, you'd know that Big Mom and Kaido own Poneglyphs. Can't acquire their Poneglyphs without strength.
Nope, just telling you the facts.
The Manga hypes him up as the strongest creature. He's lost 7 times as a Pirate if you do not exercise selective reading.
All his losses are not because they weren't 1 on 1s, that's something the fandom made up because they tend to create their own hype.
He's only hyped up as having impenetrable flesh and being a monster.
Big Mom fought him and it wasn't exactly like it was 1 sided, it was clearly even.