The only alternative I can see to having a physical button is to use up screen space and have it in the UI. It would have to pretty big too, given it's importance. That aside I think you're jumping at shadows. The dial and end call buttons on other phones seem to hold up. I've not had one fail yet, and can't remember any friends having that problem either. More specifically, for the iPhone, I don't remember Reading any reports on the iPhone forums since last October, when I started following them. What I *have* seen is reports of screen damage/failures. And more general experience, I've seen the same with other models that friends own. Even those robust stubby little Nokias.
Furthermore, it's very difficult to engineer-in a set failure point. The variance will be so wild, that to get a reasonable 'hit' of post warranty, pre-obselence failure, you'll be taking back a huge number of phones - perhaps 10% - whilst still in warranty.
The single most effective way of generating new purchases is to foster obselence through the release new models. Happily for Apple, that is a zero cost spin off resulting from the need to maintain a competetive edge in selling into fresh hands.
For all these reasons, I don't for one minute think that Apple's future sales plan is geared around that very convenient, zero-screen use, and seemingly robust button.