With the launch of the iPhone 7 combined with iPhone Forever, it's Sprint's opportunity to shine.
So you'll definitely have a bunch of upgraders from the 6s (iPhone Forever) and the 6 (those on 24 month leases or 24 month installments or 2 year contracts). Also, assuming that the iPhone 7 supports Three Carrier Aggregation (which I really hope it does), you'll have a substantial influx of those newly capable devices onto Sprint's network for that feature to go live by Early 2017 as Marcelo discussed in the recent earnings call.
In addition to upgrading its network, Sprint has to get as many folks onto Triband or CA-capable devices as possible so that they actually experience the improvements.
I sure wish Apple put Two Carrier Aggregation in the iPhone SE. That would have really helped Sprint on things, but I guess that's Apple being Apple. Hey, at least it's triband. Apple wasn't so kind to Sprint with the iPhone 5s as we recall.
When you look at these figures from Page 5 of the Quarterly Investor Update, it's interesting to see what percentage of people have devices that aren't triband or that don't support Carrier Aggregation. If you were to cross-reference customer satisfaction rates (or specific churn rates) with those customers, those would be some interesting findings. I'm sure (or at least I hope) Sprint has done that:
The good thing about programs like iPhone Forever is that it keeps people upgrading to the latest tech for Sprint's network... assuming Apple builds it in of course.