But still, what are cyclists to do when they don't feel it's safe to ride in that traffic lane?
When I was in school, we had classes that taught bike safety in school, after school, and in the scouts. We were taught how to ride FRAP and how to take the lane when needed. AFAIK, school kids no longer have classes like this.
Also, when I was a kid there were about half as many cars in the US auto fleet. Speed limits were lower and road designs were less complex. All around, it was less challenging to ride "vehicularly" back in the day.
So I have to be a little more forgiving when I see younger cyclists on the sidewalk. I hope that soon we will have bicycle infrastructure that they find acceptable. I certainly agree with you to a point, that sidewalk riding is far less than perfect. At the least, I encourage everybody who rides frequently to take the time to learn how to ride safely in the street.