Why do you think large companies like Coke continue to advertise? There are several factors, one is they want to grow their business and get more people to buy their product (no company is ever satisfied with their current level of customers). Two, they don't want a generation gap. Yes, people would still drink Coke, but would the next 20 something group still drink it if they didn't see it advertised as a kid. Especially Coke, because many parents do not allow their children to drink Soda and if they do, it is normally caffeine free type of drinks. So people don't get into the soda of their choice until they are older. So those advertisements help them see brand recognition so when they make their own choices they drink Coke. If they stopped advertising they could risk losing a certain generations business and that would be very bad for any product.
So yes, they would still have the same level of business for many years to come, but at some point it would bite them in the butt, and they would not have growth.