The truth is that, if you do the same things you were doing with your computer 10 years ago, well then you don't need a new computer!
If all you do is write books, a Pentium III will do the job just as well as a brand new PC.
Of course, the web throws a wrench in this. Word 2003 is still far more capable than Google Docs, yet tons of people opt for the cloud slop because it's convenient and free-as-in-beer. And, Google Docs will continue to become less efficient with time.
My desktop has 8 and I have no problem keeping multiple tabs open using up-to-date Firefox.