Neither company survives, if we tie survival to the lack of significant change. The market is changing at an unprecedented rate and both companies will have to adjust to the realities of the future.
But in an absolute sense, much like Apple can benefit much more from one or two hit products than Microsoft, so too can they more likely be killed by one or two failures. Microsofts complexity makes it harder for them to broadly succeed and fail because the complexity is both an impediment to growth and a protection against absolute failure.
However, as far as being at a peak as opposed to a shadow of their former greatness, both firms are exposed. Apple is too tied to their CEO right now and Microsoft, like IBM who preceded them, seems unable to compete as a company (more often defined by infighting, internal politics, and executive posturing). And it has a number of divisions that are a drag on the firms financial performance.
In a way, the risks represented by both firms are at opposite extremes. For Apple they may actually be too focused and dependent on their CEO; for Microsoft they arent focused enough and their CEO and Chief Architect likely need to play much stronger roles in ensuring the firms various units are both strategic and contributing to the companys success.
Microsoft is currently chasing too many competitors where, in more successful years, they had their competitors chasing them.
Wrapping Up: If Apple and Microsoft Die, who or what Kills Them?
Both companies have reached a level of success that would make it nearly impossible for a competitor to take them out. But both face internal threats that could cause the same terminal result.
Remember, DEC, 3-Com, and Netscape were all thought to be immortal in their time and in some cases market leading and all are but memories today because they screwed up?
For Apple they need to find a way to back-fill Steve Jobs because no other threat the company will face in the next decade will exceed that of their CEOs departure (and no one lives forever). For Microsoft they absolutely need to get back to basics and reduce the massive complexity they have created while bringing all units into compliance with high quality (in the customers view), profitability goals. They must recover their brand which has slipped catastrophically and image.
For Microsoft I think a combination of EMCs customer loyalty quality program, HPs Operations and Marketing focus, and the return of a Bill Gates-like intolerance for stupidity would do wonders. The term empty suit has been used far too often with good reason at Microsoft in current years (many of whom have been removed belatedly from the firm).
Neither company faces an easy future without their founders, but unlike Dell, neither founder is likely to come back if the firm gets into trouble, so theyd better figure it out.
HP is a good example of a firm that can actually be greater in a post-founder era than they were when the founder was there, but they went through one hell of a valley before they got to their current peak. That valley, particularly if its a death valley, can be avoided, but well probably know in 2020 whether both or either firm finds an alternative more successful route.