What if San Diego became the world's most populated city?

What if San Diego became the world's most populated city? What if all the sudden we started having tall buildings in a lot of places and had 100 story buildings in downtown? What if we had several downtown-like areas with tall buildings? What if the population was 50 million? Imagine the craze of zillions of people all over the place and El Centro being the get away destination. haha