Sidewalks weren't invented to get people off streets, either.
https://www.smithsonianmag.com/innovation/when-pedestrians-r...
Now: should our transportation infrastructure support cars? Yes. Society moves on. Should it also support bicycles and low-speed electric vehicles? Also yes.
We do not have "rads" anymore, we have "roads." "Road" is derived from "rad" but they are two different words with different meanings.
A sidewalk is a walking area on the side of a road, and that meaning of the word has not changed. Bikes, for example, or prohibited from sidewalks in most Western jurisdictions (lack of enforcement notwithstanding), and essentially all Western jurisdictions prohibit motorized vehicles from sidewalks. In both cases, it is because the sidewalks are meant for "pedestrians", i.e., people walking.
https://www.smithsonianmag.com/innovation/when-pedestrians-r...
The same "children cycling on the road" argument (which is only dangerous if the cars on the road are going faster than the children) can be applied to children walking on the sidewalk where people cycle at full speed.
2) Cars pose a risk to cyclists, even if cars are going the same speed, which is a fictional scenario, as many motorists don't go the same speed as cyclists and instead overtake them dangerously ASAP.
3) The risk posed by cycles to pedestrians is far less than the danger posed by autos to cycles.
It's not drivers who say this, but the Law. Ever heard of 'Jaywalking' Tickets?
Nothing self-righteous about that.
Sidewalks are -- as the name itself suggests - meant for WALKING on the SIDE of the road, not for riding scooters and other vehicles which pose a danger to other people walking. Only exception being people on wheelchairs, but they aren't driving at break-neck speeds like an electric scooter rider, or a bicyclist.