you are forgetting that most people are inheritently lazy and will not grow their own pot just as tomatoes are about the easiest crop to grow and people still buy them at the store.
Growing $50.00 worth of tomatoes that will last over a period of a few weeks is one thing, but growing 15 or 50 thousand dollars worth of pot that will keep you and your friends stoned, earn some gas money, and of which a portion can last for years if stored right, will be enough to motivate plenty of young men to give it a shot, many other regular gardeners will merely grow it for the beauty of the plant.
Guys being guys, and stoners being stoners, then once it is legal to do so, the growing frenzy, and one-upmanship, will take off like a rocket.
I would guess that just about anyone on this thread that has grown tomatoes, will grow some pot, even without being interested in using it.