Go look here. Look at the pictures of the Porsche 911 over the past 40+ years. The details change a bit, but you look at any one of them and you recognize it is a Porsche.
They slowly evolve and improve: more fuel efficient yet more powerful engines, better safety features, better/different finishes.
That's how the iPhone has been and surely will be far into the future. There will be little differences, curves here, angles there, glass vs metal, but the iPhone you'll buy in the future will have that same recognizable style compared to what we have now. You'll recognize any future iPhone as being an iPhone.
Nobody expects Porsche to come out with a completely redesigned and different looking new model every year.