somebody is going to want cash in
Maybe. The hard drive makers have long experience with boom-bust cycles. They are likely trying to figure out if this is real, or just a blip and in a two years (when the new factory opens!) demand drops again.
If there is new capacity in the future my guess (guess!) is that this is a case of a new factory to replace the old, and they just keep running the old for another year instead of scrapping it right away.
Get lucky - just as this was starting I saw a failed disk notification on my NAS so I ordered a new drive just before they went up. Then I realized it was a stale notification for the drive I replaced a year ago so I have a spare should one fail.