If every bank involved agrees the invalid signature is valid, what happens to the money?
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.
Anyone can audit the blockchain, not just miners.
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.
It's not just miners checking the transactions.