Plenty of other precursors to use if you can't get pseudoephedrine at scale. Trying to control ultra-small scale producers doesn't accomplish much other than sweeten the cartels (and purity/yields, so I guess that's a positive).

Opioids being more dependence-inducing than stimulants would better explain a production shift.

Big legitimate trading volumes with China (and any other large producer of goods/inputs) make a customs-focussed effort at controlling supply basically impossible.