Maybe 3 years for processing, but almost never for overall performance. My 2010 macbook cannot play games with any graphic level comparable to my 2001 Pentium 4 desktop.
What gfx card do you have in your Pentium 4? Just curious because I got a free 3 ghz p4 tower that I'm giving to a friend who hasn't too much money right now. It has a very old nvidia card though, so we are searching what's the best card we could put in it, it only has AGP though.
Back to the topic, I guess that's a good indicator, when you can't upgrade a desktop anymore (= when you'd have to buy a new component for almost everything but maybe the case). I doubt we can bring this old p4 to a level where it beats my 2008 mbp in anything, but we are hoping that we can play some more modern games together. That's still pretty impressive for a 2004 machine though.
But not everybody needs some kind of top of the line performance. 10+ years ago a laptop wasn't good for anything but simple tasks, no one did graphic design or something like that on a laptop. Today, most graphic designers I know have mbps. C2D laptops can do quite demanding tasks very well, I use mine for CS4, Final Cut Express, Cinema 4D and some gaming, and am very happy with it. Most people won't ever do such things on their machines, for youtube, facebook, emails and word you will not notice a difference between a macbook and a mac pro.
As far as I know, there are now more laptops sold than desktops, and I believe this trend will continue. The portability, space and energy savings for the loss of often not even needed performance are worth it for more and more people.
