I don't know where you're from, or how old you are, but I graduated high school before I even heard of people wearing Ballet flats. In the nineties, no one wore them- so to me, they've been out of style, they're in style now, and they'll probably go out of style again.
the same thing with skinny jeans- they were not cool in the nineties- they were considered "dated" it takes a little age to have perspective about these things (not that I'm very old)
I think you are thinking of something completely different from what I think of as preppy. When I think preppy, I think of the following elements
buttondown shirts with everything
Sweatervests
v neck sweaters.
Cable knit sweaters
loafers,
argyle prints,
pleated skirts
Collared Polo shirts
Navy blue blazers.
plaid