I really wouldn't understand such a move. If I had to do a Hangout instead of placing a voice call, this has major unfavourable implications on my telecommunications:

1) I have to ensure that I shower and shave before every such call.

2) As a somewhat average looking (at best) chap, I have to contend with bias (however subtle) against my appearance (especially when doing interviews and such-like)

3) As a, uh, "person of colour" with a relatively clear accent when speaking English, I have to contend with bias (however subtle) against my appearance (especially when doing interviews and such-like)

4) (admittedly minor reason) I would be unable to physically goof off while still sounding relatively professional (think pacing about, spinning on the chair, stretching out on the couch, etc.)

For all these reasons, I've never understood the point of things like FaceTime and Hangouts replacing normal voice calls.

The new iOS Hangouts app is a gchat client. While I'm not sure how it's implemented, it seems they'll likely bundle gchat (and other google chat services) under the Hangouts brand.

This doesn't seem like a good way to 'kill it off.' I would imagine that the vast majority of users aren't using this feature (even though they probably should).