You don't have to wait for HypeMax. WiFi has already been doing the job and is less expensive. Even with that is hard for WISPs to make financially. WiMax gear will be very expensive making it hard for current and new WISPs to justify the cost. But otherwise yes you are right. Using Wireless Broadband for the last mile in rural areas is the cheapest alternative compared to a Fiber/Copper solution.
WiFi has already been doing the job and is less expensive. Except for a couple of things:
1. WiFi requires thousands of antennas of cover a whole city, which would be a daunting task to install, let alone to maintain on such a setup! 
2. WiFi is too interference-prone from microwave ovens and cordless phones. 
Yes, I do admit that WiMAX will be initially quite expensive, but you know prices will come down rapidly and given the tremendous advantages of WiMAX over WiFi....