I think I see where you're going wrong here. We need to put some numbers to it.
Suppose you can make $20 if you spend $15 adding a feature. Obviously you should do it, right? You'll make $5. So you do.
Now the customer comes back and says the new feature doesn't work in some edge case and they won't pay the $20 unless you spend another $15 fixing it. At this point the original $15 is a sunk cost that you can't get back and you're presented with the original scenario again: Spend $15 to make $20. So now you've spent a total of $30 to make $20. This can repeat arbitrarily many times, putting you even further into the hole each time, until you recognize that hidden future costs mean that you're actually spending far more than $15 to make that $20.
Suppose you can make $20 if you spend $15 adding a feature. Obviously you should do it, right? You'll make $5. So you do.
Now the customer comes back and says the new feature doesn't work in some edge case and they won't pay the $20 unless you spend another $15 fixing it. At this point the original $15 is a sunk cost that you can't get back and you're presented with the original scenario again: Spend $15 to make $20. So now you've spent a total of $30 to make $20. This can repeat arbitrarily many times, putting you even further into the hole each time, until you recognize that hidden future costs mean that you're actually spending far more than $15 to make that $20.