We CAN see planets without aid. They look like stars that don't twinkle, so if you were expecting to see a NASA 4K image in the sky, sorry son.

Planets we can see without aid are Mercury (Very difficult to see because of its proximity to the sun and the tiny frame of time before sunrise and after sunset we have to see it), Venus (The third most brilliant object in the sky after the sun and moon. You can see it before sunrise and after sunset too), Mars, Jupiter, Saturn and Uranus. Although it is true some of them cannot be seen in certain areas because of light pollution, I don't think humans are loosing eyesight because of it, specially if you consider it has been a problem only in the recent decades, which still is too fast for a noticeable evolution.

Neptune was discovered with mathematics and confirmed with the use of a telescope indeed and followed with the roman gods name nomenclature. I think it can be seen too without aid, but only under certain specific conditions and with really good eyesight.

In fact, planet means "Erratic star" in greek because when ancient astronomers watched them they had an irregular movement along the sky. They looked like the moved forward, then backwards and then forward again (Note that this was only noticeable in a span of months and years). This movement is called recession and procession, and it is caused because of their orbit around the sun, ours and their relative position in the sky. But as you may know, they didn't knew this in ancient time, so they assumed these "stars" had a will of their own, therefore making them believe they were gods.

Source: I like astronomy