this is a odd thread but i will still anwser,,, i DO think that people will switch over to D3... but some people who are just too involved into D2 will stay for a bit or atleast play a bit of both, because some people still Play diablo1, and diablo2 is alot better, so since its better i know there will be alot of loyalists who will keep to d2..
BUT, since d2 is so good, i think people will be itching to see whats on DiabloIII because it SHOULD be alot better.
but only time will tell =D