The short answer: In my opinion, carpets today are engineered to last about 10 years, but they can last longer depending on the traffic and use. When you replace yours really depends on your own personal preferences and tolerances.
Why doesn't carpet last 30 years anymore?
The era where carpet would last 30 years, I think, is pretty much over. It's just like water heaters and appliances: they don't last very long anymore either. Carpets now are engineered to be softer, more comfortable, and more plush and luxurious. The trade-off is that you don't get the ultra-long wear that you used to get.
Is there carpet that still lasts longer?
There are exceptions. Some high-end, dense nylon carpets will perform like the older-style carpets, but those tend to be quite expensive now. (See Nylon vs. Polyester Carpet.)
So what are most people doing?
Most people want their carpet to be soft, comfortable, and luxurious. That's why they're getting carpet in the first place. So they put carpet where they want that comfort, and they put the high-traffic areas into a hard surface. At least that's kind of the current trend.
If you're trying to decide what goes where, see Carpet, LVP, or Hardwood: Which Is Right for My Home?, or come see us in Provo or Lehi.