Originally posted by Eric Lee:

But that means that it's ok to agree to arbitrary win conditions with your opponent?

“Whoever has fewer cards in their library wins this whip mirror, ok?”

“Whoever has more creatures on the battlefield at the end of turn 5 wins, agreed?”

“Would you like to agree to the alternative win condition of whoever draws the most cards during this game will win at the end of turns?”

Are these all ok things to do?

All valid conditions. Not always ideal, but valid.
The key point is these players are looking at the board state to determine who is most likely to win if there were no time limit and assuming no other cards affect the board.
Personally, I'd prefer if the debate was left to the end of turns since a player may have less life but have their opponent on a shorter clock, but that eats into the already depleted round time so I can understand that not being encouraged.