I don't think WotC would have put as much effort, hype, and time in to a product up front without planning on supporting it more than at least a year. I'd think they'd have to make at the very least a two year commitment just to get caught up on the time and money they've already got in.
Further, more time and effort is on the way. Just putting together a solid setting book, on a par with the FRCS, takes a lot of resources. Not to mention we'll probably get (as mentioned earlier) at least one book each of critters, spells/items/religion, and races/classes. Probably three or four adventures like the series that began with Sunless Citadel. Probably three of four novels in the first year as well. That's a mighty big ball to get rolling if you plan on just chucking it out after 6 - 12 months of "OK" sales (of course, who knows what the sales will be).
Finally, I don't think it's fair to compare this to Ravenloft, Dark Sun, Planescape, and Alternity. All of those were suffering from TSR's financial problems before WotC even got involved. I don't believe any of them sold in the numbers that the Realms stuff did, so TSR had to focus on what was paying the bills. Alternity (including Dark Matter) was on it's way out before the buyout, mostly because of the high production values in the series eating away at the margin.