Would this hypothetical rule change ever matter?
Here is a combo that results in one player winning and losing the game, and another player losing:

Alice has no cards in her library or graveyard. Nathan has no cards in his graveyard.

Alice controls the following:


Laboratory Maniac equipped with Sword of Fire and Ice.
Nefarious Lich


Nathan controls the following:


Grizzly Bears enchanted with Ward of Piety.
Nefarious Lich


Then the following happens:


Alice attacks with Laboratory Maniac
Nathan chooses not to block
The Sword's ability triggers. Alice chooses to target the Grizzly Bears
While the Sword's ability is on the stack, Nathan activates Ward of Piety's ability twice: once targeting Alice and once targeting Nathan.
Both Ward activations resolve.
Sword of Fire and Ice's ability resolves. The original text of the effect is


  Sword of Fire and Ice deals 2 damage to target creature or player and you draw a card.



After we apply the Ward of Piety replacement and Laboratory Maniac replacement to that effect, it becomes


  Sword of Fire and Ice deals 1 damage to Alice and Sword of Fire and Ice deals 1 damage to Nathan and Alice wins the game.

At this point, we apply the relevant Nefarious Lich effects. The result is


  (Alice exiles 1 card from her graveyard. If she can't, she loses the game) and (Nathan exiles 1 card from his graveyard. If he can't, he loses the game) and (Alice wins the game).


There are no more applicable replacement effects, so the ability finishes resolving. Neither Alice nor Nathan can exile a card from their graveyard, so the end result of this effect is


  Alice loses the game and Nathan loses the game and Alice wins the game.



This is the result of a single instruction, so it is simultaneous.