I think that'll happen around mcap = $10 T (approx. gold's mcap).

At that point, BTC would be stable enough to be used as an actual currency -- but at the same time, because it'll be so valuable and its supply fixed, I think it will always be used primarily as a store of value and pure collateral against which to borrow fiat, without having to sell it.

There will always be people who managed to have a plot of farm land in Manhattan in 1900 and then proceeded to sell it, instead of developing it, of course. I for one do not intend to become one, though.