I just upgraded my 3 year old Macbook Pro 15 Core I7, 16GB RAM, 1TB SSD with a 15 inch Retina, 16GB with 512GB SSD. And the speed difference was minimal. In fact, the older system seemed faster more times than I would have liked after buying a $2600 laptop. I'm guessing most of it is from the fact that the Retina has to push a 2800x1800 screen.
But the reason I really upgraded was ports and radios. Out went USB 2.0, wireless N, bluetooth 3.0, a 256MB DGPU and Thunderbolt 1 and in came USB 3.0, wireless AC, bluetooth 4.0, a 2GB DGPU and Thunderbolt 2 (and a better battery life). Now, some stuff, like Photoshop now being able to run its 3D features, and having USB 3.0 and its wicked fast transfer speeds, are now possible.
Processors from 3 years ago are just as capable as what we can get today, but its the "side" features than sometimes force you to upgrade.