I have never officially lost a computer. All the computers that I own still currently work, I just don't really use them. The laptop that I'm currently typing on has a few issues with it. One of these days I need to do a restore of this computer to take care of everything. I'm missing some crucial dll files on this machine that prevent me from using certain software that is on it. Luckily though, it doesn't stop me from using the internet so that's what I've been using this laptop for that mostly.
Honestly, I generally upgrade because I need a new computer. Either my computer doesn't have enough power to run what I need it to run or it's so old that I know upgrading will benefit me. My first PC is around 9 years old right now and still works fine. It would be better if I restored it, but it's still capable of handling most of what I do, or at least, it's capable of handling the internet at a decent speed.
The one I got a year ago is a powerhouse. I built it to handle the type of 3D modeling I was working on without fear of it crashing halfway through. I lost too many projects on my previous one due to crashes, so that's why, when I had the money, I decided to upgrade to avoid these issues in the future.
If you want to keep your computer healthy, you really need to restore it every once in a while. I highly recommend that when you get a new computer, make a restore point immediately. Then, install the software you want like office or other programs, and make another restore point. That way, whenever you need to restore your computer, you can either go back to the factory settings, or you can go back to just when it first stared, or just after all your software was installed.