A also have a lot of RX 470, RX 570, RX 5700, but I will not sell them now. Maybe when they are profitable again, the prices of these gpus will be a lot higher.
There will be competition from newer GPUs though? Do you think those GPUs will sell for a decent amount if the new generation is just $50-$100 more expensive? Judging from how AMD keep making their newest card cheaper, secondhand last generation card will definitely get cheaper too. If the hash/profit ratio is not better, I'm assuming buying the new card would be better no? Don't know about Nvidia though, I don't see them lowering their price anytime soon, judging from how they keep making new $1k cards even after their sharp decline in GPU revenue.
No new farms are opening up and no new enthusiast home miners are on the increase,quite contrary they are on a big decrease and as such is only natural for us to see such dropping numbers for GPU-s right now and for the foreseeable future.
For Nvidia, I also think their price turns off a lot of gamers, especially the lower to midrange segment. The performance increase doesn't really justify an upgrade from the last gen, it would be stupid to buy them if they are looking strictly for value. Personally, I'll stick with my 3060 TI for the foreseeable future if gaming is my purpose.