I'm sure the Paperwhite outsells the Voyage and the Oasis but I'm not sure that has much to do with what we're talking about. I suspect both the high end Kindles are selling enough to be worth it in and of themselves, and very likely those who buy more expensive models buy more books.
That might also be true with Kobo; that people who buy their high end readers buy more books. Another reason they might want to cut costs on their cheaper model.
I'm just an old retired computer programmer, retired now for a couple of decades, and all I know about publishing and ereading other than from my own experience is what I read in here and in various blogs. I don't pretend to have any expertise. Just lots of opinions.
Barry