Everything you say is true, including that pot isn't a gateway drug. At least, it isn't in Washington and Oregon, because of the laws against it make it a "gateway drug" because the folks who sell cocaine also sell pot. Back when Reagan started his version of Nixon's "war on drugs" the war was really on marijuana only. It got pretty dry, was hard to find for a while. Everywhere I went looking for pot the response was "sorry, man, it's really dry. I have some coke, though."
When was the last time the liquor store guy said "we're all out of beer, want some cigarettes instead?"
And then there's drug testing. I know several people who ultimately became crack addicts because their employers tested for drugs. Pot is easily detected for a month, while the cheap tests the employers use can only detect cocaine 3 days later. So folks stopped smoking pot and started smoking crack... and ruined their lives.
The government demonizes pot, one of the least harmful drugs (legal or not) in existence, so a young person starts smoking pot, discovers that the government is full of lying sacks of shit, why would he then believe the government propaganda about heroin when it's been proven they lied about pot?
So you are correct, pot doesn't lead to harder drugs. The laws against it, however, do.