I meant, one can generate such private keys "hoping" it will match an existing one then he/she can sweep it into a new wallet, eg. like an attacker would do.
You can always start with a good randomly generated private key then do some random changes using that initial key as seed. Is there any protection for such attempts?
There is a protection against it and that is the vast range of valid keys. One can search for thousands of years and still not be able to cover a tiny portion of it.
People are winning lottery from time to time...from those cases when people are complaining about stolen funds, are they really sure it was their fault of just the current design?
Finding an already used key is like winning lottery every single time forever, after all lottery is just 1 in ~10 million chance!