"You're not going to win the lotto jackpot".

[EDIT]

OK, one example was a bit flippant and easy to dismiss, but:

"No, this isn't going to bring us meaningfully closer to economically-viable fusion power"

"No, this isn't the War to End All Wars"

"No, you're not going to keep your New Years fitness resolution"

"No, your company's not going to succeed"

"This miracle chemical's probably gonna turn out to be bad for our health. I look forward to finding out how fucked I am because of it in 40 years when someone gets around to doing anything about the alarming study someone will likely do in 10 years only to be ignored. If I make it that long. But at least we can make things fire-resistant 6% cheaper than before!"

Et c.