If every bank involved agrees the invalid signature is valid, what happens to the money? Now apply that to bitcoin.
You bought into the lie that it's "secure" so completely, that you can't conceive of a situation when it's not. There are plenty, some are obvious, others, less so.
The situation I describe is almost exactly what you described. The swarm is supposed to have some people processing transactions. They confirm the keys. If enough people confirm the fraudulent signature as valid, the transaction takes place, both in bitcoin, and at your bank. You seem to understand the attack 100% and refuse to accept it's possible. It's been proven possible, and is at the point now where it's quite practical. The "fault" is that if someone were to steal 100% of all bitcoins, nobody would ever use another bitcoin. So you'd just destroy bitcoin, not gain anything. Stealing a coin here or there from a wallet that hasn't been touched in a while would be more "practical", and for all we know, is being done now. Bitcoins are finite and identifiable. It'd be possible to find every bitcoin not traded in the past 3 years, assert it "lost" then the attacker fraudulently claim them with the attack given, and it's possible he could liquidate after the theft without anyone noticing until he's cashed out.