I mean it depends on the trail, there are plenty of great cycling routes that are trails. The C&O and GAP and Katy Trails (as you mentioned) are all quite popular MUPs that people tour on. I think some short distance in city ones might not be as great if they are super crowded but if long enough there is usually a point at which heavy traffic trails off as most casual folks don't want to be that far from home or their car.

I like being away from cars while touring at least loaded down and especially on narrow roads. However yes the frustrations I think everyone shares. I enjoy touring to be away from more people for a while, get out of the city or suburbs and be somewhere else. However sometimes it is fun to have people around as I have hit the wall on tour before where I just felt alone and couldn't really decompress after a rough day with anyone in person just over the phone and it didn't quite work. However I was a bit ambitious in that trip and maybe could have planned it a little better.