So, could Hrgovic win the Rumble if we swap him in for Ali?

Those guys not being more durable is pure guesswork on your part. Just because you believe it doesn't make it objectively so. The most durable guys are generally big men by the standards of their era. So that doesn't seem to be the case.

Foremans power failed him when he came up against guys who hard to stop like Ali and Holyfield so why wouldn't that hold true in this era?

 

Fury, Joshua and Dubois are not more durable by the standards of their era. Usyk, for example, has a better chin than all of them.

Tyson was more durable than Lewis despite being 25 lbs lighter in his prime. Same era.

Lol. Ali neutralized Foreman's power with extremely good defense and lightning fast counter punching. Holyfield said Foreman was the hardest puncher he ever faced. So Foreman's "power" didn't fail him, its just his opponents had a lot of other tricks up their sleeve to neutralize his power.

 

Meaning Ali's out and Hrgo replaces him? Yes I think Hrgo could win but Foreman could possibly be the only human
that could knock Hrgo into unconciousness.

 

They might be less durable relative to their eras because guys today hit harder. Holyfield was knocked out while Marciano wasn't but I wouldn't say Holyfield was less durable he just fought in an era where guys hit a lot harder. If Marciano had to fight the guys Holyfield fought he probably has multiple kayo losses. I think Usyks is better defensively not necessarily more durable.

What makes you think Foreman hits harder than Zhang or Itauma?