Simply ending inventory co-mingling would likely handle a substantial portion of counterfeits since Amazon is sourcing their stock through standard channels.
Amazon knows this. There's probably cost models somewhere in HQ. But ending this would mean: slower delivery times, and Amazon would have to carry the inventory on their books. I'd gladly wait an extra day or two if I know the item I get is legit (just like pre FBA/3rd party days). Instead, Amazon is burning goodwill and cash on efforts like this one.
If anything, Covid-19 has forced other retailers to up their online game. Its accelerated my spending shift away from Amazon as I'm tired of fraud. Fraudulent items can injure, poison, or even kill people. Thanks, but no thanks. I'll wait another day for that delivery.