> Nobody would pay $1BN for all of them, because that would render them all worthless

Not to nitpick, but do you mind elaborating on how so? As long as there's still demand for bitcoints, you could argue that they are worth approx. infinitively more.

Bitcoins will still be a scarce good. The demand will determine if they're worthless.

A recent (perhabs a bit far fetched) example of this is the bad loans in CDOs. Although they were rendered worthless, they increased in value because those who had insurances on the value of CDOs going bad, still had to buy the actual CDO in order to exchange it for cash. This happend because more insurances existed at AIG for the CDOs, than actual CDOs - thus an increase in demand at fixed supply so to speak.