when there are 2 players left, player who resigns or runs out of time should be second and last one standing should be winner. rest is ok
For that, a player should ensure that he has the best of pieces as it comes down to the end. That will make players not trade away their pieces, which, in turn, will make the game boring, and possible never ending.
Last man standing would remove all motivation to attack, win material or trade any pieces. Any two players who enter in to a confrontation would be immediately at a disadvantage. However, the current system finds a reasonably balance between long term positional play and attacking play/material gains.