It is possible... I mentioned that. My point is that, contrary to what you've said, it is not a matter of "simply running another one". This requires a highly skilled engineer to modify code and rebuild a validator program. There are a handful of people in the world who can do this - but yes, it is possible. Throw on top of that, the odds of any one validator's block being accepted is already low due to competition (depending on how the particular chain works), secondly, being picky about block hash lowers your already bad odds of having your block accepted. So knowing all this, it is up to the developer to ensure the users of his smart contract are at least aware of these possibilities (however improbable). In my experience, small gaming systems, to be frank, no one ever gives two shits. I say, use blockhash to get things started. If you successfully build a community and there is real value at stake and real concern over corrupt validator abuse - switch to a more secure solution.