I think webOS will never disappear.
They most likely won't be a competition to Android and iOS any time soon, but for a company like HP this is not the main goal.
Android will be popular mostly due to freebies (from maps, to translation, to Voice, to Mail/Calendar, etc.) and freedom to tinker.
Apple has loyalty. The rest - webOS, WP7, MeeGo, etc. - will be valued for what they are really worth - usability...
Diogen.