Is iPhone design changing too fast?

Is Apple forced to rethink their iPhone design this year even though they (and most of us, I guess) are absolutely pleased with it, just because they have set expectations of new design every two years?

Try to understand me. If you have done something as best as you can, and with as little design as you can (as Ive says) - why changing it?

Here's a quote of Jonathan Ive:

So much of what we try to do is get to a point where the solution seems inevitable: you know, you think "of course it's that way, why would it be any other way?" It looks so obvious, but that sense of inevitability in the solution is really hard to achieve.

So I'm asking you - why should it be any other way? Should Apple, after newest design change - just stick to it, with small changes every generation if they feel it's needed or should they try to do it differently with every major release of iPhone?