[throwawayReply]

Yes, if people accept fractional reserve banking for bitcoin. I'd actually assume that's how most exchanges operate under the hood but many people who buy into bitcoin for ideological reasons won't accept that.

A consequence of making that formal is that the total owned amount of bitcoin would be more than 21m, because the hacker would own bitcoin and the users would own bitcoin on the exchange.

As long as there isn't a bank run, that discrepancy would not be a problem, but it would deflate the currency, also seen as unacceptable to bitcoin purists.

edit: access -> accept.