Sacrosanct
Legend
I think it's important to remember that lvl 1 PCs are heroes, significantly more powerful than the average person. So when you start looking at all those characters from folklore and mythology, many of them don't do much more than a low-level PC, and the legendary creatures they defeated in those stories were thus defeated by lower level PC in the D&D context.I've done CR 6 legendaries but nothing lower. I think my issue is that legendary monsters aren't just about mechanics, its got a certain "something something" flavorwise. These are the monsters of legend and myth, it seems weird that it would only be a challenge for 4 people right out of boot camp.
If you look at my examples above, one is an NPC personality and unique, thus could be a legendary creature (certainly when compared to other goblins), and the other is a legendary horse from mythology. Nothing about that horse in mythology suggests that it would be able to take on and defeat something like a giant or dragon, but it's still considered legendary.
YMMV of course.