"...but there are more chances to repress that particular under-fitness in any descendants that carry it.." And Mother Nature wouldn't care about that either,

"...but there are more chances to repress that particular under-fitness in any descendants that carry it.." And Mother Nature wouldn't care about that either, even if it were likely to happen. Which it isn't. Reproduction has nothing to do with any "fitness" other than the biological ability to reproduce. Mother Nature really doesn't care whether we think an organism is "fit", she just wants more kids. In fact, the poor could be considered "less fit" in a certain view, but they reproduce the most. Because in the eyes of Mother Nature, they are perfectly "fit". There will never be a time that Mother Nature deems them "unfit". The unfit will always have the ability, the inclination, and the opportunity to reproduce. That's just how she works. We don't have to like it, but that doesn't make it any less true.