I never said that it's fair... basically the impossible standard comes from the incredible successes attained in relatively so few years. In my opinion it's not even true that Apple is different in this respect: other companies simply had success of lesser magnitude and/or less charismatic leaders. Add on top of that that the leader tends to be considered the only reason success was attained in the first place and here you have your impossible standard.
If you're into soccer and want a completely different example look at Guardiola leaving Barcelona or Ferguson leaving Manchester United: you'll find exactly the same "fear, uncertainty and doubt" about the future being discussed.
It's not that Cook is the wrong person, it's that you come from a (false?) sense of security (with Jobs we cannot go wrong) into uncertainty. Basically every change is both a risk and an opportunity, but if you come from great successes you'd rather keep going with the status quo, which you know is successful: change is seen more as a risk. If you come from disappointing results change is seen more as an opportunity.
The truth is that change is inevitable, and even if you are successful you have to change now to be ready tomorrow, but for many it's psychologically very difficult to accept.