That's a different prediction that what was originally posited - that the marijuana users will eventually all become heroin users.
That is obviously an excessively detailed overreach of the salient concept. No, not all marijuana users will eventually turn to Heroin, but a lot of people will be encouraged by legal drugs to push deeper, and those people will eventually end up on some much worse life destroying drug.
And that statement is not intended to minimize the damage caused to people's lives from being pot-heads. Drug addicts invariably start out on weed.