The kinds of people you speak of would have gotten into 'Hardcore" drugs had they smoked weed or not. They have an addictive personality, and weed just happens to be the most commonly and cheaply available of any kind of abused drug.
The fact of the matter is, people want to get high. They get high on what they can get. Don't blame weed for being a gateway when the people would be exactly where they are now, regardless of having smoked it or not.
This is pretty much true, only that strain does have an impact on THC potency. As an example take a look at just about any seedbank, they all list the potential THC contents of every strain they sell, IE like 10-20% usually.