There is no (reasonable) way to have cities like Tokyo, Delhi or Shanghai if each family lives in a detached home & people commute to work daily by private automobile.
And this is not even tackling having to alleviate climate change by reducing emissions.
If someone pays money to register something it shouldn't be the state's concern whether it's a car or a fork lift, moped or a bike, as long as it fits in the lanes and has the right lights they should be able to drive it on any road on which they can keep up with traffic.
Nobody wants to do 40 on a moped on the interstate with traffic speeding by. Market solutions for both these problems have been proven to work well.
Likewise your personal air pollution doesn't make a perceptible difference but if everyone is driving cleaner vehicles it benefits everyone collectively with cleaner air (this was very noticable when e.g. Delhi required auto-rickshaws to convert to natural gas).
The issue with scooters is the same with motorcycles. First and foremost is safety but quickly arriving behind that is weather. Get below 70F and it can be cold even moving only 20 to 30 miles per hour then top it off with rain or worse?
The second issue with scooters and motorcycles is that in some regions they are just too easy to make off with so in the case of scooters I could see a government just using the rental system where they are everywhere giving no value in stealing them.