Jobs' replacement is probably the hardest decision apple will ever make in their entire history.
The rest of them were clearly clueless and got him back after he left.
His visionary is extraordinary, people think they just employ a bunch of clever people, they do, but jobs has ideas that are rare and the ability to appoint the right people for the job.
It was his vision that saw the potential of a GUI and pointer devices. He present new ways of selling in store and he appointed Jonathan Ive, genius and from my home town

Then of course came the powebook, ipod, macbook, iphone and ipad, all when jobs returned.
Sure he hasn't played as much of an active role in recent years with his health especially, but he is the apple brand, even if we don't know much about his past we know he drives apple forward. What we don't want is another Steve Ballmer.
Dam hard decision, but it has to be made. I'm very sorry for him and his family with regards to his health. I lost someone close to me from this recently and i honestly hope he recovers.
//Edit
I'm not sure Tim Cook is right for the job but who am i and what do i know? Well from what i know of him, his work ethic is amongst the highest in the world but that doesn't give him Jobs' visionary. He's passion is defiantly there but i think Jonathan Ive is more matched. I defiantly think the successor MUST come from inside the apple ranks. If they get someone in it will kill the apple family. Regardless of who they hire. Apple is a unique family and you can only drive it if you understand how they tick. The problem is Jonathan Ive is too talented to not be hands on. TOUGH DECISION!