There are many substances, certainly creating addiction or dependence, that all humans ingest. If all humans ingest them, then all heroin users, being human, have ingested these substances. Perhaps heroin users "start with" aspirin. Perhaps they "start with" sugar.
Where are your studies that beer, or sugar, or aspirin, or antacids, are not the "gateway drugs" that lead to heroin use? Don't their usages predate marijuana use? Perhaps one of them is the real culprit.
Sequence does not lead to correlation, and correlation, as noted, does NOT equal causation. You have to prove that marijuana use leads to heroin use. That is the scientific method.
Anything else is just ideology - ideas accepted as natural fact, but with scientific evidence to prove them.
No one is claiming that. Otherwise we'd have 97 million heroin users, wouldn't we?
The question is, is it more likely that a pot user would try heroin vs. a person who eats carrots trying heroin (as cited earlier)? Do you need a study to tell you what you common sense tells you?
Or are you going to sit there like a two-year-old in a highchair, kicking and screaming, demanding absolute, incontrovertible proof of that connection before you will accept it?