Um, actually yes. There are often empty seats on rides.
One means to fill them is called "single rider." In the before times, instead of letting a seat go empty, I happily sat next to strangers. I mean, all of us pretty much do that all day at WDW anyhow, so why not? I don't care about the picture. I can take a picture just before/after the ride if I'm so inclined.
I always saw SR as me doing everyone else a small favor, because it makes the line shorter for everyone. It is a bit like that old game Tetris. Efficiency is awesome! And I've met some really cool people via riding solo. The last time I rode TT SR, I met a senior-aged couple that were first time riders! I was able to answer their questions, ease their concerns a little. they shared their enthusiasm with me! Very sweet people! Seeing the ride through their eyes gave me a fresh perspective. I very much hope when I get to be their age, that I am equally adventurous!
As for going to WDW with younger family: if we were eligible to use SR, we did. Everyone in SR just has to be 7 or older and comfortable with splitting up. (If we had children under 7 (or not comfortable riding solo), then we split up or stuck with only FP/rider swap.)
To answer your later question, yes, if everyone did SR, the wait would be just as long as standby. If/when that happened, we adjusted, but that wasn't very often.
If many folks don't understand FP, even fewer understand what single rider is. Yes, it is kind of magical. Knowledge is magical. I hope that's why we all participate in this forum, to learn from each other. Though some days, I have my doubts.