They really aren't offering any incentive, except not being charge the $10 fee to keep the iDEN network operating. And Sprint has even said "a rate plan that is similar", which means you'll most likely pay more. I'm sure those that are still on the Free Incoming plans will be sure to leave due to Sprint NOT being able to offer something "similar". That package was a HUGE hit when it came out. Free incoming, 300 out going, nights and weekends at 7pm, for $40, unless you got them to get it down to $35 (which was a promo at one time). I spoke to several people in a Sprint Corp store a few days ago and they were STILL on that plan with Nextel's side. They were in their raising hell over the $10 charge. I left before things got ugly and later seen them down at T-Mobile signing up (this was in the mall).
I do know that First Energy Corp and some areas of NiSource still use iDEN and have thousands of phones. I'm sure they'll be one of the first to leave if they're hit with the $10 per phone fee. Also LF Ventures, aka Lowe's uses iDEN as well.
Another big hit if they have to pay full price or close to it for their fleet of delivery trucks.