Denting the performance that way makes them no longer cost effective for eth, but doesn't make them completely useless for any workloads (in games, or adobe premiere, etc) that happen to look like crypto mining - so they can avoid selling a completely broken card.

Because halving the performance makes them unprofitable to run. If you buy one you can still try mining, and if it falsely detects mining your desktop won't crash, but professional miners won't be interested.

Considering they sell the cards afterwards, I remain unconvinced that they will (to miners appear to) be unprofitable at half the hashing performance.

I fear these guys will just want the double amount of cards now :/

So sorry, but I'll believe it when I see it.