How long does an expansion “last” before it gets stale? “About two years” you say? Now what about a game that has the full leveling experience included… a game where both primary and all 3 secondary professions are meaningful?
Why do people keep rolling on “fresh servers”? Because the journey is what is fun. You have the retail mentality of “endgame, endgame, endgame” but if that is the sole purpose for playing Classic… you are right. It’ll get old after a couple years… and then you leave.
But many people love and miss the entire journey of taking a character from level 1 to 60… trying new professions, making gold, running dungeons, maybe even raid! For those people? They will roll another alt or two, or 7… they will throughly enjoy every minute of it.
And if Classic does well, there will absolutely be a Classic TBC and Classic Wrath release… with character copies to allow your Classic character to remain in Classic… but move on into TBC should you care to.
Classic cannot get additional content, because no additional content exists or ever will. Well it does… it’s called TBC.