OP is technically incorrect that rescheduling alone would solve the fentanyl problem, but not for the reasons you state.
> For one, it's much cheaper than high quality heroin. Second, heroin used to be the dominant drug when people couldn't afford prescription pills. Peoples tolerances on opioids quickly build and some subset of the population were using so much heroin that they could more cheaply inject or smoke fent. Going up that pipeline can happen in months, going down could take years.
Fentanyl is cheaper than heroin because of supply-side constraints: specifically, fentanyl is produced in larger quantities for pharmaceutical purposes, and the illegal markets are structured in a way that promotes the distribution of fentanyl over the distribution of heroin. However, that does not mean that fentanyl is inherently cheaper in an abstract sense. If legal restrictions were lifted, both fentanyl and heroin would be dramatically cheaper than either one is today.
This is, incidentally, another argument in favor of maintenance programs. Almost all of the indirect harms associated with illicit drug use, and many of the direct ones, are a consequence of the legal status and the expense (both financial and nonfinancial) associated with them.
Clinical trials present overwhelming evidence that, when provided with a low-cost, pharmaceutical grade supply of heroin, users are able to hold down stable jobs, maintain permanenent housing, etc., all things that they previously struggled with due to having to spend so much time, effort, and money in order to address what is fundamentally a medical issue for them.
> The same way prescription pills are a pipeline to fent, meth is the final stop in a similar pipeline of stimulants.
This is more or less the "gateway drug" theory, and it's simply incorrect. There is no "pipeline" of stimulants, and to the extent that one can even be argued to exist, methamphetamine is not the "final stop" in one.