Power and cooling requirements would likely prevent you from achieving absolutely insane density I think. Plus, you would go broke trying to fill the thing with $250 cards.

You could use smaller cards, but then you have the nightmare of trying to find a stable supplier that doesn't have a ton of fakes in their supply chain.

Even then, your competition is 10TB HDDs, and you need 25 cards + card readers + cooling to match one drive. Performance is better, but probably not enough to justify the expense.