The thing is... when Apple launched the iPhone, they probably already had ideas and maybe even prototypes of future models, but they didn't have a detailed roadmap of new models/designs/features. There was no plan to have model numbers or generations. I think the "3GS" is testament to that.
And so, over the last few years, as prototypes have succeeded (or failed) they have aligned new models of iPhone with an almost annual update. Sometimes they have brought enough design updates to justify a new generation number (let's say the iPhone 4) and sometimes, the new designs just didn't work out (like last year) so they brought an iterative update to their current design. (the 4S)
I don't think they have a core plan to bring out a new design every two years, with a spec update in between. I think that's just how it has worked out so far.
There must be so many variables, not least manufacturing, that determine if the prototype they're playing with at any point can go mass-market a few months later.
For example, there was clearly a teardrop shaped iPhone 5 prototype on the go last year.. but for whatever reason, the decision was not to proceed with it.. so they dumped it, and up-spec'd the 4 to 4S.
This year, they must have the go on a new design, so will call it the 5.
Next year? I don't think we can assume a 5S... perhaps there's a prototype 6 in testing now, with some crazy feature.
But know this, the decision on which prototype gets made into the marketed phone.. and what they call it... is made by high-up Apple execs, after long meetings, and much discussion. And rightly or wrongly, it's working so far.
