It's easy to fall for it: if they look at what their mining power will make if applied to the pre-merge difficulty and rewards, it probably does look okay. Of course, in reality all the ex-ETH miners doing this will make difficulty go through the roof facing a constant reward, taking them below water.
Some who understand the above may think that price of other PoW coins will increase matching increased work ("there's more work, work is value, more work higher price, right?").
Then at some point all this will unfold, taking PoW coin prices down, which will increase coin sales to pay shortfall on electricity bills, which feeds itself, so once all this is done one can expect both very cheap GPUs and cheap ETC. LOL.
Then we'll probably get a bit of bungee jumping on the share prices of AMD, Nvidia, etc as the stock market participant interpret the associated blip in new GPU sales as "the end of gaming" etc.
Most miners stuck it out to the bitter end. Hashrate was only down around 20% from the peak at the switch to PoS. Then it takes a few days to take apart the rigs and get them ready for listing. And that's only for the ones that aren't trying to mine something else, or simply are in denial.
But the main counterpoint to the GPU supply increase is the pent up demand from two years of usurious GPU pricing. A lot of people are ready to finally upgrade as soon as prices are reasonable.
Expect the market to soften - and it is, 3060ti prices are down like $40 over a couple days on eBay - but I also wouldn't expect bargain prices.
(A quick ebay search shows that 98+% of sold auctions for 3090s did so at a price of >700$.[1])
[1] https://www.ebay.com/sch/i.html?_from=R40&_nkw=3090&_sacat=0...
[1] https://www.bestbuy.com/site/nvidia-geforce-rtx-3090-ti-tita...
No, 3090 didn't make sense to use for mining. Too low profit compared to what you had to pay for it. There is better cards to get for mining, even if they have worse performance. Performance per $ makes the rules.
But it's going to take them time to get them prepped for sale, so they're not going to show up all at once.