I completely agree.

There was no RTS that could beat Warcraft II until.... Starcraft.
There was no RTS that could beat Starcraft until Warcraft III and thats debatable.
There was no ARPG that could beat Diablo until.... Diablo II.
And IMO nothing (and I've tried them all) can beat Diablo II until.... (hopefully) Diablo III.

Diablo II was my default game until I started WoW. I played it almost exclusively to all other games for about 6 years. I took breaks for a game here and there, an FF or Bioware game would come out and I'd drop DII for a month or two but then I'd come right back. If Blizzard would have kept making expansions I probably never would have left.

WoW is now my default game. I'll take breaks and play other games from time to time. Dragon Age and Torchlight will be my next two breaks. As soon as I get tired of themone it will be back to WoW until the next game distracts me for a month or two.

I think there are only 2 things that can kill WoW.

1. Blizzard stops making expansions.
2. Blizzard comes out with something better.

Although I believe that 2nd one will be extremely hard to do.