Pot is a gateway drug for some people, just because it is about as low in chain as you can start. If there wasn't pot, then something like acid, shrooms, or whatever would the gateway drug. Some people just can't control themselves though, it wouldn't matter if pot existed or not, they would be on meth or oxy regardless.
It's like calling 16Gs a gateway turbo, they were for a lot of us, but nowdays people are skipping right to 61mm turbos and wanting to go up from there.
While some people can keep their speed habit under control and stay with a 16G for years.
I don't think bums drink instead of smoke pot because of cost. Pot isn't expensive. $100 worth of pot can last a long ass time. Alcohol is pretty expensive, unless you are drinking Boone's Farm and alcohol and pot are somewhat opposite types of stimulants.
Alcohol and cigarrettes are as much of a drug as anything else. They kill people using them and often end up killing people who don't use them indirectly. But they are already big business and legal, so they aren't considered taboo by our society.