SC1's longevity lies in competitive multiplayer. Unless every episode brings something new to the multiplayer, I don't see much appeal of this for the hardcore group(mostly fans in Korea and China). A lot of people don't have the patience or just don't care enough to play through all the missions for one campaign, let alone all for the 3 races.
That said, I don't doubt the power of Blizzard fanbois. Even if the difference between each product is only in single player, I suspect Blizzard will still see huge success for each release.