Even if they're dead? Keys can be mined with computing power, miners run data-centers, it makes sense that miners will claim old keys since anyone can do it.
And I trust mathematics/physics. You are and will be unable to brute the private keys with current tech.
No kidding. I'm not talking about current tech. How long will the 160 bits hold? not forever, and we'll have to upgrade. If someone doesn't upgrade what do you do?
The 160 bit hash is just an extra layer to protect against a potential flaw in secp256k1. When you transact, you reveal the public key anyway (that's why we have change addresses).