Oh I agree, but from recent sale prices it looks like they go for more for whatever reason. Certainly only getting 5-600 for the 30xx makes the math worse. Probably wouldn't go through the trouble then!
Most threads I see people say the 3090 is still perfectly fine, which it is of course -- but it also looks like my 3090FE would go for $900+ on ebay, which is still $800 after fees. Maybe 'only' $700 on forum like this.
Even with the 40 series...