Over $60 billion wiped off value of cryptocurrencies as bitcoin drops below $8,000 again....

If you’re talking about the GPU silicon, it’s much better and safer to run it 24/7 at a moderate temp than to cycle it from cool to hot many times a day for the same period. The heat cycles are more detrimental than actual usage. Even so, the majority of failures will be on the package connections to the board, less on the silicon itself. Mining cards typically are underclocked and under volted, but the VRAM is typically OC’d to the moon, you might see degraded lifetimes there, but DRAM can take a LOT of abuse before anything bad happens, you will crash before you reach settings that cause irreparable damage to the chips.

The only silicon that degrades over time directly from use alone is NAND flash, which GPUs have none.

Fans on the other hand, will degrade and eventually fail with constant use.

I’d buy old mining cards all day as long as I could source new fans in case one fails.

You're missing out on a lot of details about what limits the life of ICs. Thermal changes, cyclic heating and cooling put a lot of stress on the chip and solder joints, particularly when that gradient is asymmetric. The very act of pushing electrons through gates and traces does very little in terms of wear to the device. This is why sticks of RAM can last decades. A constant load/temperature device is going to wear more slowly than one that is cycled and pushed to 100% several times a day.

Bitcoin means nothing and has no value if no one is buying anything in cryptocurrency. 99.9% of vendors actually take it. And why would you spend it if you think its value is increasing? You would hold it, logically. And why would a vendor take it if its value can decrease overnight? Crypto as it is now DIScourages people from spending it and using it for its intended purpose.