Relase value and fragmentation are not the same thing. If you think that you're going to sell a 5 year old computer for anything close to what you paid for it, or that a 5 year old computer should run the same OS version as a brand new one, dream on. 5 years is a LONG time in the ever-evolving computer world. According to Moore's Law, the number of transistors on a computer chip doubles every 5 years. A typical video game console generation lasts 5 years.
As far as 5 years of use, that hasn't been a problem for any Apple product I've ever owned. My 3rd gen iPod Touch doesn't run 5.1, but it still works. It still does everything it was advertised to do when I bought it. I'm happy. I got a good six years out of my Power PC before I felt I needed to upgrade to an Intel MBP.
Plus, there is a huge second market for Apple products, and they do typically hold a reasonable value for used, outdated tech. In fact, I'd be willing to bet that they hold their value better than any PC. That doesn't mean that Apple needs to update the OS for these old Macs. People who buy tech on the second hand market shouldn't expect Apple to keep them updated with the latest OS.
Fragmentation is when multiple devices currently sold on the first market are not keeping up with the latest and greatest OS. I'm not talking about older, first gen Androids that aren't sold anymore. This is about multiple Androids being sold brand new right now that each run different versions of the Android OS. It makes the Android community and marketplace more complicated, and creates different user experiences. People can argue all day about which method is better, I'm just saying that that isn't what Apple is doing by not updating the OS on some old PowerPC. You might as well say Ford is fragmented because some 1975 Mustang doesn't have an iPod port and a sunroof.
Oh, and resale value in no way factors into my decision to buy technology, although I understand why it does for some. I generally don't sell old tech. Typically I either keep it or it breaks. I use my Apple devices professionally, so the factor of resale value is easily trumped by the factor of how much it will help my company grow and make money. Resale is part of YOUR Apple experience.
But either way it's irrelevant. I certainly had resale value in mind when I bought my 2006 Subaru Forester. I'd like to see it hold it's value, but I'm not going to get all upset because the 2008 model was redesigned with an iPod port and in-dash GPS. I don't expect Subaru to come to my house and install those things on mine.
What you're talking about simply isn't the same thing as fragmentation. I understand your frustration however. My 3rd gen iPod Touch would no longer communicate with my MBP, because I upgraded iTunes. The latest version of iTunes won't speak with the outdated version of iOS on my iPod. I didnt get mad. The iPod is probably four years old. It still works with my PowerPC G5. Eventually I got an iPhone.
I sold the iPod Touch, and boy did it keep it's value.