They just don't though.
The Phil Neville model pro stuff is irrelevant. You say he is 'undoubtedly' fitter than Hibbert, but he's just come back from knee surgery and runs like he is under water most of the time.
Hitzlsperger's fitness is constantly improving, he was out of the game for a long time. Even if he's not playing 90 minutes every week, how has he only got one season left ? He's a better CM than Neville all day long, and is five years younger.
Hibbert is 31 and has had some injury problems as of late. So has Kevin Mirallas. When it comes to actually playing football he's perfectly capable and i see no reason to suspect he can't be a functional squad member until he is the age Neville is now. Where have you gotten the indication he can't be ?
To say Neville and Hitz both have the same shelf life is madness.