What kind of traffic are you going in?
I just ride in bike lanes and don't worry about cars. I don't think that I can reach 50mph even on the best mountain bike in order to stay in traffic. If I go downhill, I prefer just to coast looking for hazards and saving power for next uphills.

If you have a coaster brake, you can use your hand brakes as well. 3 brakes are better than 2, aren't they?

Ramps and pins in cassettes and chainrings are the things that help to shift. Single-speed sprockets and chainrings don't have them, and you can turn them upside down when the teeth wear out from one side.

Chains mostly wear not because they move but because sand and grime collect in them and act as abrasive material.
Full chainguards can protect from it.