There is a big difference between last "chance" and last "push" and history shows you can NEVER count people out.
Look at Mark Henry, he had been on the roster for 15 years when he finally won the big one and had had countless pushes in that time, ranging from Sexual Chocolate to Hall of Pain.
Some guys are realistically on their last chance... someone like Swagger or Ziggler is in that category... if they screw up again, they're gone, not demoted but likely fired. Someone like Orton could also creep into that category as his 2 strikes still stand... but during that time, they could all still receive several more pushes.
It's a misnomer to put Barrett into this category... first off calling him injury prone is innaccurate. It is more that he has been taken advantage of by some of his peers at crucial moments. Both Ziggler and now Swagger have benefitted directly from injuring him and them taking the push intended for him. You only have to have seen Triple H's impression of Barrett when introducing him as part of that MITB match to realize they are very high on him... guys like that who have genuine "bad luck" aren't a problem... he could get 3 more pushes over the next 5 years or no more... but they won't fire him cos he's a good hand who is useful down the card as well as up top... and more than willing to do whats required... he hasn't blown a push yet but others have been careless and benefited as a result.
There are some who are very much on their last push... Kane is for sure, by bringing him back to "The Demon" they've now gone full circle as much as they can, cos Taker clearly isn't gonna be able to put him over in that last match. Brock is arguably on his last push... he'll win the title and it'll be his last run with it as if things go as WWE hope then the Shield, Bray and Cesaro will be carrying things going forward and he won't be needed.
Some guys won't get another push intentionally... ADR is a classic example, he is being slowly phased out as he has made enough noise about not renewing his deal that they will maximise his "putting over potential" now... Big Show won't get another one now unless the Shaq match ever happens, but he's not going back to the main event any time soon unless it's to be fed to Reigns. Rey is not going back into push and if he's disputing the renewal of his deal by not cashing cheques, it's doubtful he'll be seen onscreen again.
Some guys like Sandow, Ryder and arguably Christian are perhaps in the worst place, unlikely to get another push and very likely to be released before they get another chance. In Christian's case it's not his fault, but 3 concussions in a year takes you out... no messing. Ryder is STILL suffering from going into business for himself all those years ago... Vince doesn't like guys getting themselves over unsanctioned as Ryder did...even if WWE DID base much of their strategy for today from it.
There are some guys who will likely get more chances than others over the next few years. Cody has done the right thing, he's realised it's not happening for him so he's looked at Dustin's career and will go for that instead...longevity over titles... by creating a character that will sell merch, even down to gloves with the star he can secure his income in a WWE without the level of PPV money once enjoyed by the guys in the ring. Stardust is hokey to some, but it's a gimmick that can keep Cody employed for another 10 years in WWE and right now, that's very important indeed, far more than if you ever get another push. No one saw Dustin ever getting another hire much less becoming a cornerstone of the roster and massive push... but no one can say it's undeserved either.
It's the one thing nearly all these guys have going for them... they have major faith in themselves, even if Vince doesn't always... few actually screw their pushes up the way Ryback tried to or Swagger managed to weeks before Mania... but even if they did...if there's money in it and the timing's right, it would be stupid to discount them or ignore them for a past mistake.