1) Prices aren't returning to "normal".
The only way they will is if the hyperscalers and AI companies start to implode -- which will kill a huge portion of the US economy and lead to global recession, so, cheap RAM but nobody can afford it
2) By building up capacity you influence the outcome.
If someone else enters the DRAM space, the duopoly has to actually start thinking about competing on price, maybe they become price competitive before the launch of your new fab in order to kill it, but, it will have an effect and probably before it even opens
3) A western supply chain has benefits by itself.
There's a reason some industries are not allowed to die, most notably farming- because security and external pressure are concerning.
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Realistically there's no reason not to do this. It will be long, painful and expensive. The best time was a decade ago. The next best time is now.