so... if we aimed all the Hashing power of the Bitcoin network on one address it would take 500 million years? Hey man It's only a matter of time before insane quantum computers just start cracking the codes by the second. I can imagine this will happen one day, when the Bitcoin network migrates to a new protocol based on quantum security. Those computers will tear Bitcoin apart when they finally are able to produce them in mass, and start migrating all the accounts to a new system like a block reward, Or just like the free market migrate by choice to something safer.
I'm quite sceptical that quantum computers are physically even logically possible to build since the theory is predicated on the flawed Copenhagen interpretation. So I'd say bitcoins can't be counterfeited based on a brute force computing approach given what I have read here.