I think it could happen (
40k being dethroned, I mean).
GW's business practices are so bad that there's no way they'd survive in a truly competitive market. They rule because they hold a virtual monopoly.
Think about the rage posting you see with each codex release: "OMFG X UNIT IS SO
OP F*** THIS I'M QUITTING" but because they love sci-fi wargaming and there's no alternative (warmahordes is a SKIRMISH game, not a wargame) they end up crawling back.
But if someday there is a viable alternative released and
GW happens to have given a bunch of customers the finger yet again at exactly the right/wrong time, they could end up with a serious fight on their hands.
Speaking of which, is anyone else excited about warpath?
http://www.manticgames.com/Sci-fi.html
THE SQUATS LIVE! MUAHAHAHAHAHA!