Theory is all very well.
I, OTOH, am in the thick of it. Meth and heroin are not only going full blast but spreading. Legal mj does not diminish the use of “harder” drugs, in fact the reverse. A drug culture of anything goes, stone yourself out of your kind miasma encourages people to try anything.
Before mj was legalized, the county I live in was already a huge doper paradise, it just got even more so. Heroin and meth have increased for years, along with pot. Drugs are drugs and many people try mix and match. Once getting “high” (I call it getting “wasted”) is the goal, many people will try other stuff, as regular mj use gets boring and people have to use more and stronger to get the intoxication level they want. So many go go for other harder stuff too.
In Colorado other drug us is up too since mj legalization. Regarding no changes since legalization, it depends on location in the state and what a person runs into - personal experience, or reads. Your SIL may live in an area that hasn’t changed much. Overall, CO has had changes, not for the better.
Ah, so TheStickman's SIL knows only her personal experience of her area of the state, but you know what's true "overall"? Pray tell, how did you come by this superior knowledge?