My last GPU cost me 299 and I sold it for 250.-, so 50.- for 2-3 years.
My current GP cost me 550.- and I'll sell it for about 700...750.-. I also mined around 400.- on it. So I'll end up in a huge plus (600) there.
If I can only mine around 1000.- on the 3090 (50-100 days), including the resell price 2 years in the future, I basically paid 0.- for the then last 6-7 years for my GPUs. I don't consider that as being dumb. But who am I to judge, you seem to be the one loving to do the judging :-)