Bismihi ta'āla

Although a believer must never put himself in a position where they are criticised, and the criticism could be valid, there are things one cannot avoid. 

If you have tried to explain that throwing away good food is rejecting the bounties of Allah (kufr al-ni'mah), and there is nothing bad about eating something from the day before, then you have done your duty. 

There are people who just do not like eating leftover food, even if it is good. That is their preference, and you can do nothing about it. Of course, if they are "wasting", then this would be haram.

Many find it very normal to eat the same food for a few days, and there is nothing wrong with that, and it does not have anything to do with being kind or stingy.

What you prioritise is your choice, cooking daily, or eating from yesterday's food, both are personal choices and depends of each individuals choice and circumstances. The point is that if someone chooses to eat leftover food, it has nothing to do with being stingy.

And Allah knows best.