When/if Diablo III comes out, I would guess that the first few months will get heavy gameplay. Players on Diablo II will be reduced a noticable amount, but there will still be those that don't have the money to afford the new game or have the system requirements it takes to run the new Diablo. After the first few months are over, some people will dislike the new Diablo (always happens to somebody) and they will return to their favored Diablo II. However, many people will stick to Diablo III, since it is brand-spanking new, and they are just tired of the old Diablo. In my opinion, Diablo II will never turn out like Diablo I - it will always have players in it.
Diablo I is still a great game. It's not as complex as D2, but it has a much different gameplay than D2, whether you feel it or not. For example, there arent even boots, gloves, or a belt in D1! There was no stash either (you could just save items on ground in town on single). One thing D1 had over D2, was spell scrolls, elixers, maximum attributes, and APOCALYPSE. D1 was also much more easily hacked, which turns away a lot of players (I don't blame them). D1 also seemed a lot more eerie and evil, something I want to see more of out of the Diablo franchise.
Diablo II, I think, is NOT as good as D1, but the multi-player is far better. There aren't nearly as many hacks in D2 than there are in D1, but there are some problems with duping. As long as there are patches that limit certain items (selling SoJ's for DClone), things will stay stable. However, having items like SoJ's and such, creates a market-like environment for Diablo, creating a sort of currency.
Diablo I = Never gets old
Diablo II = Multi-player will never die
Diablo III = We'll see