If Apple ends up tumbling down in the next, say, 5 years, with Cook still at the helm, what would that say about Cook as the CEO? To me, Apple seems to be in the Microsoft's home-run stretch where they dominated the market with 99% desktop penetration for a while, but no clear "where next".

Apple certainly won't fall because of their sheer size and momentum (just like Microsoft never did), but we look at things differently at that scale and for a while, Microsoft was "stagnant" and "backtracking".

I don't know how people define "most successful CEOs of all time", but I do know that predicting future is a fool's errand (you can go into statistics and risk evaluation, but here we are talking about one person thus not much room for "statistics": I mean, everyone who's hugely successful but then hugely fails has been mostly hugely successful right until they failed).