A Pentium 4 or Athlon XP 2700+ from 10 years ago has been unusable for all but the simplest tasks for years. It would have also needed to be upgraded along the way - it came from the factory with just 256MB or 512MB RAM. You can still edit office documents and do some light web-surfing, but for todays interactive sites and content it falls far behind the experience that an iPad can provide.
My mom still has no problem using QuickBooks and doing all the required web stuff at work with a Pentium 4 and 512mb ram. She literally just bought a new desktop because quickbooks requires it.
In an office environment, she really doesn't need more.
I'm not so sure. A Chromebook provides a significantly better experience than an iPad, at least for me and my wife.
Our iPad 2 is now almost exclusively the province of my toddler daughter.
PC sales may be mostly done. A desktop that was bought 10 years ago can still run fine today - and many still are.
We reached a point in computing where upgrading doesn't make that big of a difference. You can use the same machine for a decade or more without problem if you're an end user, especially given so many rich web apps that don't require high processing power.