yes, while the show probably popularized the idea of purity for meth, in general strict prohibition leads to increase in purity and potency. We've recently seen that with heroin/fentanyl. There is probably still no "fentanyl of meth", and thus so far only purity increase. Once a more potent, fentanyl-like, meth appears, it will probably similarly get into and displace a lot of classic meth trade.
What? Prohibition historically showed the exact opposite.
I suspect higher purity & potency of street drugs has much more to do with more sophisticated operators operating outside of the US than strict prohibition. Same with fentanyl.
I believe OP wanted to make the point that one of the most important things for people profiting from the illegal sale of drugs (meth or heroin/opiates) is to minimize the amount that has to be trafficked (1kg of 10% meth vs 100g of pure meth or 1kg of heroin vs 10g of fentanyl).
I believe this explanation is too simplistic...
yes, the "Iron law of prohibition" https://en.wikipedia.org/wiki/Iron_law_of_prohibition
"the harder the enforcement, the harder the drugs."
The production case for a stronger stimulant is weaker. Heroin is a really complicated molecule. It is only made from a natural precursor. Meth can be made by two major pathways, and P2P can be made by at least four off the top of my head. It was the fentanyl equivalent for cocaine. For anything else, you balance the increased complexity of synthesis with any increase in potency.