not sure where this is coming from. you do realise in MM's on SC we usually run 100-110 vitality so at level 90 we'd be over 9k health even in the back ranks. The only mobs which can get remotely close to that damage in "one hit" is the triple attack mobs which also presumes all 3 attacks crit and i'm still pretty sure that's way short of 9k.
This is hardly "any monster" also the chances of all 3 hitting the same backrow target with all 3 being crits is near 0.

not really sure how you personally would know much about level 90+ mobs, perhaps you made it once many years ago and your memory isn't as fresh.

My comment is based on experience, level 85 with 11,000 hit points and got 1 hit damaged of 9,000 hit points (collectively), yes it was 3 normal monsters at the time, luckily I exited after using Salvation... I did not even have the chance to attack when they spawned... I was left with 125 ish hit points, luckily the second phase was not a Critial Hit... the first was was Critical hits taken.

NOTE: People dismiss monster having the damage capacity of HEAVY weapons in their arsenal, on top of critical hits, on top or Multistrike abilities in conclusion of Added cold or other elemental damage(s).

What would happen if you character is like level 65, puts a White level 55 Longsword into a Well and then got 1 hit killed because the drop (if you could kill it and not die) would have been 2xx% EE and it was a Critical Hit and on the max damage hit chance... do you think you would survive a 20,000 damage of hit points... even at level 90... I am talking solo... so group yay, but at least 1 of your 5 would get insta killed right??? And yes... I have seen it happen on Hardcore with a full group @ level 35 at a well dropped in a level 45 earth charm (main drop) and he got killed instantly... the rest of us was lucky to survive being killed because everyone exited while I sat there healing myself and the rest.... the charm (main drop) is a 163 or 165% EE Earth that I still own to this day... and in those days, we had to BRIDGE in hither level characters to kill off that monster.... are you saying monsters are easier now from updates???

Yes, it is like a 1/1000 chance it could happen, however, it doesn't account for the fact that IT CAN HAPPEN.