Another thing is why doesn't Cobb care?
He just went through all that hell to see them. He pulled it off, he won, he did it, mission accomplished, so Saito makes a call and he's good, right?
So then even if he is asleep there in the kitchen, and the top keeps going, couldn't he just blow his brains out or something, like with Mal and wake up knowing for sure he can go see the real kids.
But like the guys in that basement, or like Mal or Saito at the end even, I think he's just lost a handle on reality and is afraid to find the whole thing was a dream, with no deal, and that if he doesn't just take that chance right there, he may never get another. Like the way he's offered the job and left them to begin with.
"they come here to be woken up, the dream has become their reality."
Cobb is scared, he still doesn't trust his own judgement and hasn't all along which is why he cannot build. Nothing has changed to reassure him even with Mal gone and the mission complete. And so his only real option to not live in fear is to just let go and embrace whatever kids those are as his reality.