I liked your question and will try answer it from a different angle. Basically we all agree that when a player fold his/her hand, he/she cannot win any pot.
BUT, one of the major concepts of the game is that one can only win according to what he/she risks. That's why there are side pots, and I think that's why the question was asked - if a player risk enough chips as an all-in player, he/she should be able to compete for that pot
I think that the rules was defined as they are today, because it gives more options of play and more flexibility to the game. the following are some examples:
1. give advantage to players with big stack by forcing others out of the hand.
2. give a chance to small stacks (when they know there is a chip bully in the table).
3. knockout games (some of the money is given to the a player that eliminate another player). etc...
As I wrote in the beginning, I think the rules are as they are for a good reason, but this theoretical question is quite interesting.