I think this is very subjective.
Unless you are actively following some people, you won't really know what they are up to. MM and Tommy are the only couple that are constantly in the media and that's literally the only reason people know what's happening with them.
For example, if you aren't following people like Wes, Josh Denzel, Kaz (s4) etc, you would say that they haven't been successful post LI. Same goes for someone like Scott who co-founded a very successful PR agency which even manages recent islanders.
Alot of people have done well after the show but because they aren't in the media all the time or you aren't following them, you wouldn't actually know that.