I do web development. Yes, computers need to eventually be replaced, but this depends on how you care, for your computer, and what you do with it.
I would never consider using mac. NO WAY!!!! Not, for web development.
I could tell you what I use, but you would never understand it, and I know, for a fact. Most people use Mac, and Microsoft Windows. Because these platforms is what's being sold in the market, but this just means that if this is all you know, then you are certainly not a talented web developer.
If your computer is not broken, then you don't need to replace it. I have had a desktop computer, for about 14, or maybe 15 years before it stopped working. There were a lot of problems with it, and it was an old computer, so I decided to through it out into the garbage.
If you take good care of your computer, then it should last you a long time.