I did buy multiple thousand dollar Dual CPU G5 machines, the day before they were discontinued and for a considerable time after, as I had an existing infrastructure which was based on such machines using proprietary software which was not available for x86 compatible machines.
That's great that your parents only need to use TextEdit, and are not connected to the internet.
I guess I should have learned though... I mean when my beloved Motorola 68k based Macs were phased out, I gladly spent way too much money upgrading to 603 and 604 based machines. Only to find that for some reason despite being modern much more powerful systems, real world performance was strangely lacking. Hmm what was going on here... oh Apple introduced a new architecture but didn't rewrite the core of their OS which was written in assembly for the 68k. Rather they just emulated the old 68k on the new chip, wonderful idea right?
Then as those machines (which I was never happy with) aged I watched MS surpass Apple in almost every category imaginable. We received some BS cobbled together upgrades which were given major release names but were just desperate attempts at keeping System 7 somewhat relevant in a world which had long since passed it by.
Still I continued purchasing and using Apple computers. Now we're back to the beginning, where we were speaking of the last generation Power PC processors. End result... In my long experience with Macintosh computers they do not "have useful lives longer lives". In fact they have shorter useful lives from what I have experienced. There's a pretty interesting Youtube vid of someone upgradeing all the way from the very first Windows up to Vista (I believe) in a virtual machine, and also showing the surprising amount of compatibility between early programs and later OS's. Stick to your guns right? One architecture verse three, that alone says something.
I've read what you had to say and besides having purchased a mini for your parents see no facts in your statement, instead quite a few opinions (including your rather outrageous XP statement)... If you wish to contradict my belief that Apple computer's do not have longer lives than alternatives please feel free to give facts and reasons to support your belief.