GSM/GPRS/EDGE (2.5G) should be around for a while still. There's rumor (RIMxxx8217;s 2011 BlackBerry lineup | BGR | Boy Genius Report
) of RIM coming out with a new Curve, Bold and Storm later this year. It will not only continue to support GSM/GPRS/EDGE but also UMTS/HSPA (true 3G) which is AT&T's stepping stone towards LTE (4G). Getting to LTE should take a while. Even if carriers get to LTE, I think it'll be another while before they give consumers LTE's full bandwidth/speed.
Frankly, it doesn't surprise me that the 8900 is at end of life given the 9300 (which also does UMTS as well as GSM/GPRS/EDGE). The 9300 shipped with OS 5 but can be upgraded to OS 6. The rumored new Curve should be designed for OS 6. So logically RIM would functionally stabilize the OS on the 8900 (maybe the rumored certificated 126.96.36.1999 release) then move on. But the 8900 should continue to work fine until its hardware gives out.
I read the specs on the rumored new devices. No wow factor in any of them (to me at least). I figure I've got time to decide what will eventually replace my 8900.