I hope I am not cherry-picking, but I looked at historical ETH revenue data. Let's assume one bought a Radeon RX 570 card for $180 of capital cost ($130 card + $50 rig) and depreciated it over 3 years, while power cost was 7 cents. I also assumed the depreciation + power can be deducted at a 20% tax rate. That is $0.33/day of total expenses per card.
In 1H 2020, the revenue for that card was $0.60/day while Ether price was $200 on average. The cost to mine 1 ETH was $150, while the cost to buy it was $200. I also used data from the entire year of 2019, where it was also 20-40% cheaper to mine the coin than to buy it. Even though revenue was 25% lower than 1H 2020, video card were cheaper too. That same RX 570 would only cost $150 ($110 card + $40 rig) in early 2019.
If you bought a Geforce GTX rig during the bear market and mined, like I did, GRIN/RVN provided double the revenue of ETH. The return was higher than an RX 570 mining just ETH.
I already have spreadsheets showing that the ROI from 2019-2020 was 20-40% per year. That is a phenomenal return compared to the stock market. That's why when RTX 3060's cost just $200 or RTX 3080 12GB's cost just $500, I will buy as many as I can. It's most likely going to be much cheaper to mine altcoins than to buy them, just like with Bitcoin.