The prolonged use of ANYTHING is never ever good, adult or child. Chocolate, salt, alcohol, sunshine you name it. So to answer your question, of course the effects on young people are detrimental. What you can’t seem to explain is why the current policy on weed hasn’t stopped this. And then trying to justify the status quo.
As for your second paragraph, legalising won’t prevent those effects. Just like legal alcohol doesn’t stop alcoholism.
Nothing will stop that.
In relation to harder drugs and the ‘stable door’ as you put it, we could then properly fund our naval fleet, customs and revenue, gardai, cab to help reduce smuggling (it will never be eliminated but we could make a big enough dent with a well resourced force so that it becomes cost prohibitive for scum to import heroin and coke because those drugs HAVE to be imported…they can’t be grown here).