Anyone in the world could crack your password. (Well, any of 2.5 billion people with an internet connection.) Requiring a physical key instead cuts the attack surface down quite a bit. If you can secure your car and house keys, you can secure this.
And of course you need a duplucate for the key.
Not only infeasible - physically impossible, in fact (barring quantum computers). Just 128 bits of entropy would take 1e16 (10 quadrillion) years to brute force at 1e15 attempts per second. :)