I would say better training and shoes. I believe under 2 hours will be achieved somewhere in the world in the next two years. I also believe the ultimate limit will be 1:30. This will be a combination of training (rest being the import factor) and shoes. Very firm shoes at that.
you know 1:30 would be 3:26 per mile? So in future they will run faster than the current WR by more than 10 seconds.... 26.2 times in a row?