This question already has an answer here:
When I read about the risk of using the blockhash as a random number it was explained to me the issue was that a miner can trow away the block if the block-hash is not in his advantage, and keeps mining till he finds one that is.
However if a miner trows away a block, to keep mining for a more favorable block, he would only have till the next person finds a block right?
So this would only be a issue if all miners are trowing out all the blocks till they get a favorable block, otherwise a other miner will mine a block in about 20 sec.
Even when the attacker would spin up 10.000 GPU instances at for example amazon, he would only have about 20 sec for him to find a favorabel solution before a other miner finds a solution?