hard drive costs halve every 2 years or so.

Where did you get halves from? Maybe if you're buying refurb/low-cap/shuck-drives on sale...? Not even the 2 year price projections (which are usually extremely optimistic) are anywhere near halving for higher-cap drives.

Even now, the only thing you'd get even nearing the optimal $10/TB mark would be a shuck-drive on sale as far as I can tell. Whereas the cheapest non-shuck is like $12.50/TB, but you'll most likely be wasting tons of time RMAing it within a year anyways because it's Sea*ate 🤢