The Mercedes-Benz S-class is a better car than the Model S in many ways (not all). A lot of people could afford either but buy the Model S because it is electric, not just that it happens to be electric.

Just a quick thought from someone that actually went shopping and drove both: The MB S-class beats the Model S in terms of luxury feel. Hands down. The Model S is faster. And quieter. But it does not feel as good as a MB at all. Before you ask, no, I ended up buying neither. I bought a Subaru Forester and an airplane for the same money.