They would be doubling their MSRPs, not the street price. If a 1080Ti goes for 1200$ with an MSRP of 750$ while still selling out, then a 2080 that’s 25% faster will sell at 1500$ with an MSRP of 1500$. The difference is in who gets to pocket the profits.
On a related note, DRAM prices have more than doubled since their previous lows.
It’s not useless, it’s the proof-of-work for the system to fulfill its job. It’s what makes cryptocurrency like Bitcoin possible.
If there was significant demand for certain work (e.g. scientific calculation, rendering) that was (currently) worth more than proof-of-work for cryptocurrency, that’s what the GPUs would be used for instead. In practice, distributing work in a trustless and (relatively) low-bandwidth environment is difficult.
We already had two crypto mining booms, prices went up, cards sold out, the bubble popped, prices went down again. This bubble is much larger and longer, but it’s still a bubble. Production will catch up, mining profits will sink, demand will go down, prices will go down. Just be patient.