Let me start by saying I have no issue with Youk at all and am not advocating a trade.
But I have always had a concern about how well he will age. Not a major concern, but noteworthy. I mean the man stands in the absolute limits of the batters box to give himself extra time. What happens when he looses a little reaction time? I mean he can't go any further back in the box to make up for lost bat speed. I'm not saying he will become completely inadequate at the plate. But if he regress's even some he will be an average 1B as far as offense goes. And many might feel different about trading him.
I don't believe any player to be "untouchable". But with Youks current production and contract, I don't see a trade happening at the moment. As far as down the road? I could see to some scenario's leading to him being traded I suppose.