Well, they've survived without Steve so far, so as long as they have a strong team and culture and vision intact, they'll survive without the others as well.
Ive is the most irreplaceable one, IMO.
Steve has been gone for a year. Nothing Apple has done thus far has been devoid of his influence. Not saying Apple is doomed without Steve, but we just haven't gone long enough to REALLY know what an Apple without Steve behind the helm is capable of (or not).