Sprint thought they knew what they were doing. On paper it sounded like a match made in heaven. They would scoop up Nextel's cash cow subscribers, and migrate them to CDMA. They made a big mistake when they migrated Boost mobile prepaid customers to iDEN shortly after the merger, maxing out the capacity of the existing Nextel towers and alienating Nextel subscribers. They ended up having to invest in additional 1800 iDEN towers nationwide to keep up, but they still ended up losing millions of subscribers. I believe a lot of those fleet contracts switched to Verizon, but the feds still use Nextel.
1) Push-to-talk was iDEN only
2) Nextel was charging $70+/month before that was a normal price for service
3) The US govt was (and still is?) a large customer
I still think you're right though; I don't see them adding a third line of phones anytime soon.