I think that Apple has always played catch up. There have been better-spec'd computers, higher capacity MP3 players, bigger smartphones, and desktop-level tablets, but every few years, Apple has made a successful consumer-friendly version of each. I don't know why people have grown to expect frequent innovation—much less be disappointed when there isn't.
To be fair, the iPhone was quite innovative and ahead of the game. I can remember what every other smartphone in the world (ever) looked like before the iPhone was unveiled.
I think that's where the expectancy comes from, but I agree it's unrealistic to expect something of that scale and significance every few years (or maybe ever again....Apple is already unusual in coming to market with more than one big innovation during its lifetime).
Actually, your comment could broadly apply to basically every tech company. Microsoft has spent the past decade coming out with their own refinement of something already done by someone else (search engine, MP3 player, mobile phone, tablet, game console). Google definitely didn't come up with Android out of thin air. And let's not get started on Samsung.
What are the real innovations of the past decade? Apple gave us the iPhone, which changed mobile computing forever; Google innovated with search and web services, which changed the way we use the Web forever; Microsoft could make a claim with Kinect, although that didn't catch on or have anything like the same kind of impact as the iPhone or Google Search. You have to go back to the 1990s for the last time when Microsoft really transformed a market. And, that's it. Nothing else has really fundamentally changed since a decade ago, except for being made a LOT faster, smarter, bigger (or smaller) and generally refined to become much more consumer-friendly. Even the most recent so-called innovations, like voice input and using a stylus and wearing a 'smartwatch', are old ideas which are being refined and (vastly) improved.
Which means I agree with you, except that your statement should be applied to everyone and the industry in general, not Apple specifically. I think that's really how innovation works, most of the time - gradually, in small increments, not through lightning bolts of brilliance from one company or individual. The iPhone was an exception, though, and it created this sense that Apple may (at any moment!) come out with something which changes the world.