That's a good question. I guess that there would be many competitors who'd one make a more generous offer to the miners than the other. They could reach paying half the reward to the miner, just to consider their transaction valid and not the others'.
And I have a feeling there are machines watching the mempool for these keys.
But if you feel you're wasting time and money on the puzzle 64 just ignore it! and focus on the puzzle 120 which you are more likely to find and there is almost no risk of attacking for it.
Or even better: Do something productive and stop searching for tricky ways to make you rich, which are highly unlikely to succeed while they're costing you much of your time!