I think it's way too early to tell. In fact, I doubt we will see if they truly will succeed without Steve until a new emerging technology trend threatens to dethrone Apple from one of the categories they dominate.

If, in 5-10 years the iPhone becomes irrelevant or outdated because of a new contender and Apple has to play catch-up, then I think we can say that Apple is in trouble without Steve.

Until then, Apple just needs to keep doing what they've mostly done even when Steve was running things, which is continually improving their products to remain competitive.