Eight years ago I bought a Dell Studio XPS 8100 desktop for a home computer at a moderate price (NZD 3,100). I've just replaced a failing 1TB hard drive with a 500GB SSD, but other than that I've done no upgrades. What's interesting to me is that it's still a perfectly good machine: quad-core i7, 12GB RAM, NVIDIA GPU with 2GB VRAM. Everything I do, this machine could still do well, including software development for work. I guess if I wanted to play the latest AAA game titles or use a 4K monitor on it, I'd be unhappy, but I can't think of anything else I'd even consider doing that would be a problem, and those could be addressed by upgrading the video card. If this machine doesn't fail catastrophically I can see us continuing to use it for many more years. (I run Linux on it; the situation might be different if it was Windows.)
This is interesting because up until 2010 I'd been in the habit of upgrading computers at least every five years because they would improve dramatically over that time in ways that mattered to me. That stopped happening. It hasn't entirely stopped for everyone — Mozilla developers are getting new desktops with double-digit numbers of cores to speed up Firefox builds — but I run my heavy-duty workloads in the cloud now, because really big machines aren't efficiently utilized by a single developer. I guess the economics of utilization and colocation will making cloud-based heavy lifting (not necessarily public clouds) increasingly prevalent over time.
One of the implications is that declining desktop sales don't necessarily mean declining desktop usage. I think they must at least partly reflect longer upgrade cycles.
Another implication is that component reliability for desktops is becoming more important. It doesn't really matter if parts wear out after five years, if you're going to replace the whole machine before then anyway. If the expected lifespan of a machine is fifteen years, it's worth buying more reliable parts.
Another implication is longevity bottlenecks might shift to relatively minor features like what types of USB ports your machine has. I guess some of this can be alleviated by upgrades and dongles but it's worth thinking about.