Looking not too far into the future, I'd expect to see most laptops/desktops ship with "smallish but sufficient" SSDs, as increasingly content sits in the cloud and is consumed on-demand. In this scenario what does Worldo do about the performance test? I appreciate that SSDs are not going to challenge HDDs on price per GB for the foreseeable future, but OS sizes are not increasing inverse to the reduction in cost per GB.

What you're looking for is hardware that proves the miner is a human intending to mine; a "human proof of work", if you like. "Something that has the performance of a flash drive relative to a spinning drive" is a poor indicator of this, and will become worse over time. Botnets aside, virtual instances have access to large amounts of RAM relatively cheaply, enough that 8GB is probably not a big deal. I suspect that cloud mining and dumping to fiat for immediate, if cynical, ROI have a bigger effect on CPU coin prices than botnets do. I might be wrong.

If you want to keep the "USB as proof of human" concept then, as a thought experiment, you could mandate a dongle which all miners require in order to function. This would be cheap to make (a few dollars in hardware costs, something that won't threaten early-term mining ROI as ASICs do) and do something that botnets or cloud compute services can't competitively do. Ideally, the dongle would be easily available from a broad plurality of manufacturing sources. I believe this line of thinking is what lead you to arrive at the "8GB flash" idea. I have no better solution, but I suspect yours won't work.