Although I personally agree cryptocurrencies have major appeal, we must be realistic here. Bitcoin is probably here to stay, etherium has its benefits, and maybe two or three other altcoins will survive in the long haul. The rest will die a quiet death.
Additionally banks and other big (groups of) companies are already actively developing their own blockchain technology, so the classic minable cryptocurrencies are already getting competition from the establishment.
So lets say 4 or 5 cryptocurrencies are here to stay, they all have a finite number of coins to mine and some (like BTC) are not interesting to mine with gpus.
So even if there are some cryptocurrencies to stay, the mining will decrease in the long run.
So if they believed the increased demand would be sustainable they would push foundries to invest in additional production capacity (which they haven’t). The production roadmaps are planned so far in advance because foundries have to allocate production capacity for customers and don’t like downtime.