You could always add a hard drive (or replace the existing one with a larger one) and add RAM to your desktop, change the graphics card, etc. Until recently, when it just got too slow to bear, I was running a core 2 duo 2.3GHz desktop with 4GB of RAM. (And about 3TB of drive space total, booting 4 different OSs.)
You can't yet do that with a phone. Yet. When modular phones come out, we'll be able to change the 3GB RAM module for a 6GB module, the 32GB storage module for a 128GB storage module, etc. Then we'll be able to keep phones until they just don't make larger modules for that phone. (Although my spare phone is a Motorola V551 - about 10 years old - and it still makes and takes calls, has a limited number of internet apps [browser and email], takes pretty decent pictures. But it's not my main phone.)
As far as the amount of improvement between generations, there are two things. 1) They're going for more flash (water-resistance, which means nothing, but it sells, "Turbo" charging, which does nothing but kill the battery faster) and 2) there's a point of diminishing returns. Once, all you had to do to double the speed of a chip was shorten the paths and make them narrower. When they're narrow enough, the path is narrower than the wavelength of an electron and, even though it's a conductor, it won't conduct, so you can't get speed that way. That's one of the reasons for multicore CPUs. (Another is that the faster the clock speed, the more heat you have to dissipate and even a CPU designed with high temperature semiconductor material running at 120C would be a bit warm in your pocket.)
SpookDroid, Moore's law says that the number of transistors in a dense IC doubles every 2 years. People have used the wording changed all sorts of ways - the power changes, the speed changes, every year, every 6 months - but Moore never said any of those things.
Of course, the speed at which technology increases, itself increases. (The increase in technology is somewhere between a 2nd and 3rd order function.) So it's probably closer to 6 months than 2 years now, but it's the number of transistor junctions that can be packed into a given space - and we're running off the end of the dock. There's a minimum number of molecules needed for a transistor. The next "quantum leap" (no pun intended) will probably be quantum computing, where the spin of a sun-atomic particle can denote a bit.