It is unnecessary to protect against an attack in which an attacker buys all Bitcoins and deletes their wallet.
Assuming we were all willing to sell, the attacker would just give us large amounts of cash for our Bitcoins. We could then start another Bitcoin block chain and start over from square one (only with a much larger confidence in the market, since it worked once!) and reinvest that money if we wanted.
Essentially, no matter what happens there is always the nuclear option of “starting over.”
There are much more dangerous and likely attacks to be worried about.