"Of course, they never state that such crap exists mostly because possession of cannabis is verboten."
They never like to admit that bit. A lot of that synthetic stuff is pretty bad but it is a problem that could be solved over night by legalizing a few of the common and (relatively) safe drugs.
Yes, they would cease to exist shortly after legalization. Unfortunately, the State does not care and just wants to guarantee there continues to be dragons for them to slay. Otherwise, it gets harder to justify their budgets in the hundreds of billions.