Those are not nice things. Double decker trains take longer to load/unload than regular trains for only a small increase in capacity. Single deck trains can make more stops in the same amount of time thus serving more people, or they can take less time in the stops thus getting people where they want to be faster. Time is important to humans, anyone who says slow down to others has no idea how they live or where their needs are. If you want to slow down and smell roses that is fine: go to a park and do so - meanwhile a lot of people need less time on transit so they get more time at home with their kids (or whatever else they do in life)

Larger loading gauges are a good things for a lot of reasons, but the ability to run double decker trains is not one of them.