That argument doesn't sound well founded. The booming marijuana industry and its popularity among previously non-pot smokers should've been your first clue.
(1) Marijuana is not nearly as bad for you as tobacco or alcohol, whereas I think we all agree hard drugs are worse. This study from a few years back should put things into perspective [1]. I think it's disingenuous to compare a drug that's objectively safe to one that's broadly recognized as objectively harmful.
(2) As of October 2018 (shortly before legalization) 47% of all Canadians have tried pot. [2] Now that it's legal, people are just more willing to fess up to it. Again, it's nothing they couldn't get before if they'd wanted it. Further [3] shows that the percentage of people smoking pot in Canada did not in fact increase after legalization.
From Statistics Canada: "For example, the vast majority (98%) of those who have never consumed cannabis indicated that they would not use in the next three months. In contrast, most daily or almost daily (93%) and weekly (84%) users thought they would continue to use in the next few months at the same frequency." [1]
