If you are taking photos that are not repeatable, anything you do to decrease the likelihood of failure is worth doing. Having 2 card slots capable of duplicate storage does that. Yes, card failures are rare, but if it impacts your bottom line, paranoia is justified.

If you are taking casual photos, you can probably do without the dual slots.
If you buy a camera for specific technological reasons and you don't need dual slots but the camera has them, you are not required to use them both. Pick one.
It's up to the individual user.

Don't be concerned about that at all. The 21 megapixels of the D500 and D7500 are more than enough. I think the pixel count got as high as it is today for marketing reasons. I bought a D500 and was glad that Nikon went from 24 to 21 megapixels. The individual pixels on the 21 MP sensor have almost twice the surface area of the 24 MP sensor. That's good for a variety of reasons. The sensor with larger pixels can have a better signal to noise ratio. Camera shake is not as evident on sensors with large pixels. Sensors with small pixels are affected by diffraction sooner when stopping down the aperture.

Another good thing about going with a D7xxx series or above Nikon is the pentaprism viewfinder. The D3xxx and D5xxx series cameras have a pentamirror viewfinder. The glass prism in the D7xxx series gives a larger, slightly brighter view though the viewfinder.