If you use or sell a ton of GPU time, hedging against future increases/decreases in cost per time unit is what futures are actually for. Cash vs physical settlement is irrelevant. Speculators assume the risk on the other side, typically. Nobody has enough margin in their futures account to corner the market on GPU time and there is typically a position limit for futures contracts (aside from market makers, they’re allowed unlimited futures contracts to allow hedging)
They do rot while in use. Thermal fatigue and other things like tin whiskers. I've heard they remove them from service early to resell them before they become obsolete and before actually fail because of these effects. Somewhere around 6 years I believe. But you're right that just sitting around unused they don't really rot except to depreciate.