Well not zero, just below half the mining costs, right? As long as it's below that, you still lose more taking over the new cryptocurrency than you gain. And that's also not taking the price drop post-takeover into account, which affects how much the attacker could actually cash out.

But yeah, it would seem that miners of new cryptocurrencies should expect to lose money initially.

That is ignoring people who do it for the lulz or to prevent competing coins from popping up.

Well, not quite - the 51% attack is not free. There is some value that makes the 51% attack too expensive, and game theory, yadayada.