Even though we both can't predict the future I'm certain that *much* more players will buy all three games for multiplayer. It just doesn't make much sense to pigeonhole themselves into the first game only. How many people would buy only the first game or first and second or all three of them? I think the safest bet is that to play with much more gamers one would certainly buy all three games. My comparison generally still stands with WoW. Lvl 60, 70 and 80 characters don't meet much (if at all). They all play different battlegrounds and dungeons. But we all know that there is only a handful of lvl 60 and lvl 70 gamers on a server. I bet >99% of players always play at max (80) level which is completely understandable. That is my point with with SC2.
Yes, you can try to quote Starcraft vs Brood War players, but frankly I would like to see some source of that. Even if the the ratio is much more favourable to SC, I still think that its not very relevant to compare a not released game with a very old game released when gaming industry was just a little more than in it's infancy.