The cost has always sunk. The fallacy is assigning it value.

In the broadest sense the fallacy is about not wanting to waste past effort. Whereas in reality 'wasting' past effort doesn't cost anything but wasting future effort does.

Now it can be hard to tell to what extent your past effort actually brought your goal closer, so it can be hard to judge if continuing effort may or may not be worth it, but if you're arguing it is worth continuing because you don't want to waste past effort, and not because the goal is worth the remaining effort required, then the reasoning is fallacious.

Sunk cost refers to whatever resources have already been spent. Where it becomes a fallacy is thinking along the lines of But I’ve already invested this much money or You know, I’ve been working on it for all these years, so I CAN’T quit now.

What we’ve paid into it is gone — a sunk cost — and what matters is how much additional investment from where we stand now will be required to complete the goal and whether that cost will be worth it when considered against other opportunities for the same time, money, and attention.

In some sense, we start at even every day.