I really am not sure what you are referring to there. Both the Kindle and the nook release firmware updates (what you call software). The upgrades for the Kindle that require you to purchase the new model are things that are beyond firmware - ie moving from domestic to International coverage or getting the new Pearl Screen. Those are hardware issues and to get the newest hardware you have to buy the newest hardware.
You don't have to buy a new Kindle or buy a new nook. Both work until they die. However there is going to be a nook 2 sometime ... soon-ish and I assume it will have upgrades, most likely the new Pearl screen. It cleared FCC approval last month.