Swinger rarity...

I don't know why you think a Dart Sport is "more desirable." Only think I like about the sports over the "regular bodies" (Dart/Swinger/Scamp) is the larger rear tire openings.
I think its more of a what would you have kept. The sport had what a 360? Swinger a /6 or 318?

I bet more swingers were sold like the one my grandma bought but more sports were kept and saved and resold.

I could be wrong about all of that though!