A lot of us are still on unlimited plans, and there is no discount for not using an upgrade there. The Next monthly fee would simply add to your monthly bill. The mobile share plans would also be significantly more expensive based on the amount of data used on my plan. I know that's not true for everyone, and a lot of people switched.
Thanks b1llj0hn for the tip about using Apple's site. The date does show there when making a phone purchase. It is weird that AT&T no longer gives the date for future eligibility on their site.