Tough sell is perhaps a bit narrow minded wording  - its just far less positive than it was just a few months back.  I didn't realize they changed the sub 8g plan credit but since I have been at >10g for quite a while on a multi-line family plan, it doesn't apply.  Using your formula comparing what I signed up for earlier this year compared to now:

50% Edge, $25 discount for >10g

Edge payments $300($12 x 25) - $300(12 x $25 discount) = $0

75% Edge, $25 discount for >10g

Edge payments $450($12 x 25+$150 to reach 75%) - $300(12 x $25 discount) = $150

If I want to upgrade every year, it costs $150 more per line to do it.  Or I have to wait 6 months longer.  That's not the end of the world but it is meaningful on a multi-line account.