Erm, actually, I think they measure success by how many books they sell.
As I've already pointed out at least once in this thread, me voting 'Never tried 4E, all earlier edition play' doesn't mean I'm not buying the books. I'm buying them even though I don't currently play 4E and probably won't for quite a while.
I doubt I'm the only one.
As I've said in other threads, I think 4e will be made or broken depending on long term success.