Reason to go back to d2 is try hardcore mode out and try to get rich and then use ur softcore dueling experience to pwn them ill experienced duelers in hc.
d2 will die when the draft comes back in and everyone has to go to the military other than that d2 will live forever because there are always new people discovering the D2 world and its many adventures of repetitiveness. And through all this many new games will come and and will fail to compare to the easy play and easy-to-achieve riches of D2 that keeps many people playing for years and years and years.
I think D2 will last all the way till D3 comes out. Unless blizzard shuts the servers down before then to make room for WoW. I would think the latter would be more likely. They make shit on D2 already and WoW being a pay by month thing is worth a whole lot more to blizzard.