I think the cardio of today's fighters would look a bit diffrently as well, if all fights did not have rounds, like the fights back in the day. If fights had a 15 minute time limit, with no rounds, you'd probably see a lot more stalling/clinching, and more gassing as well. A lot of today's fighters can prepare/train to go balls out for 3 rounds with a 1 minute break between rounds, but I think the pace would have to change, as well as the level on intensity if this was a straight 15 minute fight.
With that being said, I'm sure most of you remember some of Tanks' fights, it he went over 3-4 minutes, you better get the oxygen mask out. Even Vitor, when he was a heavy weight, didn't seem like he has the tank that he does now, the Couture fight was an example of that.