I used to say that the only differnce for projectiles in a vacuum and in
air is that the horizontal force is either present or not. But from now on
I will be adding that the net vertical force is influenced by the horizontal
component of velocity when air is present. The idea that trajectories can
be viewed as superposition of two independent motions is certainly a big
oversimplification. But we must start with simple situations, and I agree
with your quotation about idealizations (from Geore Polya). " ... Nature's
complexities would bury her lows too deep for man to probe." We must
simplify and be aware that our models have limitations.