Gareth Gates wasn't on X Factor, he was on Pop Idol. Add to that, he didn't even win that show. However, he did manage a decent career for a short while.
Bad example, really.
As for the others; I believe Shayne is still producing records, but isn't very well publicised. He's still signed, though. So that leaves Leon and Joe. Leon quite clearly flopped on SyCo, but JOE?
How can we use Joe as proof of anything when he hasn't done anything yet? He might flop, sure. But he might not. You can believe that he'll flop and that's fine - it's your opinion. But it's not proof of anything.
It also depends on how you define "flop." Most seem to define it on how big a public profile you end up with. If you're not in the papers every week, then you're a flop. Doesn't matter if you actually made a decent career and are making more money than most of us do in our daily jobs. In that respect, even Leon isn't a flop, unless he's gone back to working in the Gap?