It sounds like your solution to this is to make people brute force their own private keys, which is far from optimal.

- you seem to have not read the paper. The brute force requirement gives you the economic security against the attack.

 are of low entropy and much easier for an attacker to guess/hack

- maybe relative easier but still unfeasible to guess. I bet 1 btc, you or anyone else wont be able to hack this.