Having to deal with a drug dealer that wants to also sell you actually addictive drugs is the gateway.
Marijuana is addictive though. Maybe not as addictive as some other things, but pretending it’s completely non addictive is disingenuous and misleading. It’s more addictive than say LSD or psilocybin for example.
That isn’t to say it should remain illegal though. Legalisation has positive benefits even for harder, more addictive substances than marijuana. See the history of alcohol prohibition for example, or the disaster that is the war on drugs.