Can 18-300mm replace 16-85 and 70-300 combination?

That's not exactly a difference in crop factor. However, it is probably a characteristic of the 18-300mm, like the 18-200mm, that at shorter focusing distances the effective focal length becomes shorter than the zoom setting, which is only accurate with the lens focused to infinity. Most likely if your test subject was a row of houses instead of a row of cards, you would find that a house fills the frame just about the same with a given focal length setting for either the 70-300mm or the 18-300mm.

That effect of focusing distance is a difference between the lenses that can be important to some and not so much to others.