No, not at all. I just meant that if you feel 8 years is a reasonable lifespan for a computer, you could upgrade to a new one today and enjoy all the benefits of a current model while knowing that your needs would be met for many more years. In fact, that is why I decided to buy a new computer for a server instead of investing any time or money in my 6 year old MBP.
I replaced my 2008 MBP with a new MBA in 2011. I wasn't planning to upgrade that soon, but I was having issues with the MBP and thought it made sense to get something newer, faster, lighter with better battery life. I was just amazed by the perfomance difference - same experience as the OP in this thread. It ran Final Cut Pro much better than the old MBP for example. My 2013 MBA is even better. That was an "upgrade of opportunity" though. Again, I wasn't really planning to upgrade but a friend needed a new laptop so I sold it to her really cheap as a favor and was able to get 8gb RAM and a 512g SSD, both of which were not available on the 2011 MBA.
I have plenty of old computers - a PowerBook G4 in the closet and a PowerMac G4 up in the attic. The G5 died in 2008 and I didn't bother to fix it because the 2008 MBP was so much faster.
I have been using Macs since the 512k in 1985 (Apple ][ before that), so I "get" what old computers are capable of. But I just don't want to bother with the old stuff anymore, it's too much trouble.