ICQ was dominant in its heyday, but the game wasn't won yet, so it was theirs to lose. In some sense that's still true: we have another 4 billion people yet to join the game. But we're nearing the endgame and as we get closer to the "everyone already has a good messaging solution" line it's going to get harder and harder for an upstart to come in and eat everyone's lunch.
¹ http://ec.europa.eu/competition/mergers/cases/decisions/m184...