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.