You don't start eating 50kg a day if you become vegetarian, and you don't spend 30 hours a day travelling if you do it by train. Yes, universally switching from cars to any other form of transport would save people 1-3 hours a day to do other things, but if they spend that time doing anything other than sitting in a car it's a win.
You also don't get to use "what if noone did that" as a counter argument for something helping if everybody did it.
If everyone insulated their home properly, got rid of their cars, stopped eating beef, and cut the remaining animal proteins by half we'd be pretty close to net zero right now.