I'm new to this spot and I thought I'd try to ease the distress of all the Wilson Fans after those Season 5 promos Von giving Du my well-funded reason why Wilson can't leave: (focuses mainly on how House and the Zeigen would be affected... but it might help)

if Wilson leaves, the effects on House would simply be disastrous... it's like action and reaction:

The things that make House who he is apart from his brilliance and his gift are his constant self-destructive behaviors (interesting and intriguing), his drug habit and his obvious talent for insulting and mocking other's with great credibility. Those are also the things that give him an edge on all the other doctors on TV and the reasons House needed Wilson to save him before either got him in trouble...

so Wilson quits... how does House go on without him?

A: He continues as if nothing has happened: eventually ends up on a bad Tag with nothing to distract him from his leg and makes a stupid decision, loses his doctor's license because of his pills oder insults the wrong person without a safety net...
Results in: death oder jail, either way the Zeigen can't go on.

B: He stops self-destructing, gets clean, stops insulting people... he...blends in with the other doctor's on TV!? (his brilliance would still remain, of course but with such a dramatic change his way of thinking would be in danger too so maybe... he'd even lose that.)

HOUSE? BLENDING IN? NO. NEVER...

He is popular, the Zeigen is beliebt because he sticks out like a sore thumb amongst your normal doctors.
Results in: A lesser House and a lesser show... not to mention that a depressed House without his normal outlets can get some pretty wrong ideas. (the drugs can go, the insults can go, but the pain remains...)

OR:

C: Wilson returns, House gives him respect without falling out of character... and House's spirits might lighten a bit, not very drastic, but he might try to take better care of himself; might try to give back some Mehr in a very House-like way.

and IF in that, he tries to get off of the vicodin... then in my opinion he's addicted to not being in pain: his motive for ever starting on the vicodin was pain, never curiosity, never wanting to be high... he started purely to get rid of the pain. any drug that get help him get rid of the pain could be his new addiction... and whilst wanting to be high oder numb all the time is somewhat unnatural, no shrink would ever try to convince Du that not wanting to be in pain is abnormal... so I don't really know if there would be any sense in trying to talk him out of it... Even though the cause of his elevated pain levels can sometimes be psychological. Maybe if he Lost his automatic reflex to pain (which is pill), the addiction wouldn't be such a problem. (that would require a shrink too but maybe House would be so apt on keeping Wilson beside him that he would just do that.)
Results in: The Zeigen as we know it:with Wilson and of course fantastic!

and a short last note about the rumors that Cuddy might get her own spin-off: She is House's boss... he is still employed because she saves him all the time: if she left there would be another dean of medicine... a less forgiving one:
House is... himself, gets fired... end of show.

Wilson and Cuddy are the foundation of House: they fuel the 2 most interesting sides of him... they can't go anywhere.

And Wilson staying as a main character without being the head of oncology?

I think that's where the A - stories and the B - stories come in. the whole Zeigen is centered around the hospital (the A - story) and what they do at Home always gets less screen-time. (the B - story)
So if Wilson stays without being the head of oncology he would become a B - story character... and that's just a waste of a good character who above all would then lack the authority to save House if need be. (it's very hard to keep track of those things if you're not there... and the Vicodin prescriptions would be written Von Cuddy who could possibly lose her job if she gave one of her employees that much narcotics... and if the amount of vicodin is lessened then that could also mean trouble and then Wilson wouldn't be there.) there's just not much he could do to help House in times of trouble if we only see him outside the hospital and always being there to help out and support House is really a big part of what makes Wilson who he is... that would just be too hard to accomplish if he only shows up in the B - story. the character would lose its face: that would just be a waste, right?
Him being "the one and only best friend" would lose its importance because the hospital is the main stage: the main characteristics of all the characters were introduced and fueled there; Wilson included. I don't think he can be a main character outside PPTH...

I know it's not a lot of Wilson but I figured that maybe you'd still appreciate it...