Why do you think John sees a need for healthier menu items in the
market described above?

The linear perspective changed the quality of art because it gave art a sense of reality. it gave the illusion that the background and surroundings were actually close and/or far and a sense of depth. linear perspective change the quality of art following its birth the renaissance period as they want the art to be more realistic and they believe one way to achieve it is to use the method of linear perspective. it uses the principle of math to realistically portray space and depth in art. 

Idk but maybe a camera?

Answernon-religious themes became more popular, and people also wanted to show off their wealth because painting was a sign of this at the time;