Why, though? Lifetimes are dictated by the physical processes that govern wear, and computers wear out fast: we pump a whole lot of electrons through them, and because we want them to be faster and faster have made them less and less fault tolerant under heavy load.
Make no mistake: ever since the AT/XT days the useful lifetime of a computer has been in the three year range because of the nature of matter and the physics we exploit to make digital circuits do what they need to do. Can you still use a 20 year old computer that runs Word Perfect 5 on MS-DOS 6.22 and isn't really made to do much more? Of course. And of course, you'll still be able to run OSX Panther on a 2005 MacMini for years to come, but you can't use those things for anything new or modern, you've created a little time machine that can only do what we wanted to do in 1985, or 1995, or 2005, instead of in 2015 (or soon enough, 2025).
Most of us want to use those new programs that rely on new algorithms that require the faster hardware, or old algorithms that massively benefit from faster hardware - we want spreadsheets to take less than seconds, not more than minutes, for ever-more complex work; we want games that run smoothly, not "not at all" or at 1 or 2 frames per second; we want our internet browser to start when we tell it to, not after a period of time that is long enough to wonder whether it even started at all.
Things all have lifetimes, and for computers that lifetime's about 3 years, and has been for decades. On the upside, computers are ships of Theseus: most of the time, we can just swap out broken parts for new parts, including faster parts, so really the lifetime of a single, unadulterated, intact computer is three years, but with a few hundred bucks every three years, we can keep pulling it back into the modern age until at some point nothing of the original remains - and the lifetime of THAT computer is much, much longer.