So you have to be able to tell the future? Things can change dramatically overnight, even more so after 6 months, a year etc. Changing jobs, starting a new hobby ( in music editing, video editing, 3d modeling ), changing careers , starting to travel so you need a beefier mobile computer - all of these things can negate all of the research that went into the previous computer purchase.
For example, my current job is not even slightly similar to what I was doing just a month ago. I needed a new computer to be able to do it because I could not upgrade my old one. If your life doesn't change much, then yes, a well-researched computer purchase will probably last you for 3+ years.
That is the risk you take buying anything, you can only make an educated guess based on your current and projected needs and adjust when you have to. You could easily buy a small town car and then need a pick up in 6 months time, most things aren’t upgradeable. Sometimes you just have to sell what you have and buy what you need because circumstances change out of the blue, that’s life and a hit you have to take.