The problem is that pretty much the whole point is to go faster than walking speed. If you can't, what's the point?

The "not having to walk" part is still kind of handy even at walking speed. My joints aren't what they used to be.

most roads are safe enough to go on or have bike lanes.

i only go on sidewalks when I need to. So scooters are still worth it even if you are safe on sidewalks.

I'm not sure that's the widespread behavior though. (And I agree that partial cautious sidewalk use is fine.) A lot of users seem not to want to use roads or even non-separated bike lanes period.