Well, the purpose of nLockTime isn't really to literally lock coins for a particular time, it's intended as part of contract negotiation to allow you to use the memory pool as a kind of race resolver. Whilst you can lock some coins for a long period of time, indeed the risk is that somebody finney attacks you. You can reduce the risk by requiring both parties to sign for the output, for example, but there isn't really a good way to do what you want short of a hard forking rule change. And I don't see any use cases that would justify that.