That’s just silly.
Most leftovers if refrigerated promptly are still “good” for at least five days.
I’d say fish might be more like 3.
There will be texture differences in leftovers right away for things that are deep fried (which is so unhealthy better to skip altogether anyway).
Many dishes are actually better on the second day, although I’m not sure they get better after that. Examples would be the lasagne you mentioned, beef stew, things that the flavors meld after a day or so.
I have almost always lived alone and it’s hard to cook one traditional type meal for one person so I am almost always cooking multiple servings so I have prepared food available for a day or two. I often cook big, then freeze servings in one or two serving sized containers so I have fast and easy food available when I’m busy. In fact, tonight I got home late and had a roasted chicken breast with fennel and orange that I made a few weeks ago and had tucked away in the freezer. That with a microwaved potato from my neighbors garden and green beans from mine, was an excellent meal that took about 10 minutes.