One way they seem to make the hardware products become defunct is by releasing operating system updates that don't support earlier models of hardware. In less than two years, for example, the iPad 1 no longer is supported by Apple. They seem to also holdback features that clearly could've been put into previous model, just so they can sell the next version seven months later.
Arggggggggggggggggggggggggh!
Right, that's better. Sorry but I really hate this way of thinking. Yes, the iPad 1 (in this case) is no longer getting iOS updates. That doesn't make it an obsolete device. It still works perfectly well for many people and if you don't have a need for the functionality (and apps) that are supported only in iOS updates then it will keep working as long as the hardware lasts. As for holding back features: buy a device based on what it has, not what you want to be added later. Anything new should be considered a bonus.
To the OP: Apple builds its products very well and has a solid support network in place in the form of its Apple stores and on-line / phone services. There are some lemons of course, that's pretty much inevitable in any product line, but they're pretty good at taking care of them (speaks the weary voice of long experience

). They go to reasonable lengths to support their hardware with new OS upgrades and are pretty good at not screwing over the customer. In the mobile phone market they're probably the ONLY manufacturer that really supports their hardware properly, largely because they're just about the only manufacturer that considers you to be their customer, rather than selling to a carrier / network. By and large if you're happy with the performance, storage etc of a device when you buy it you'll probably get a good few years use out of it.
The major stumbling blocks however are, in some sort of rough order:
1) Battery life. The bane of mobile devices since some bright spark first put a handle on a monitor, welded the guts of a PC to it and called it a 'portable'. Batteries only have a limited lifespan before they can no longer hold a worthwhile charge. For laptops Apple do a replacement service, for portable devices... well you're probably better off upgrading when the battery dies on you.
2) New features / hardware. In the traditional computer market this isn't a huge problem any more. It's a mature market and even a basic Macbook Air has enough computing power inside for most users. In the mobile markets though we're still early in the arms race and you can expect raw computing power to take big jumps in the next few years (as well as other changes such as screen and battery tech). Just compare the performance and size of the iPhone 5 to the iPhone 4 and you'll see what I mean. Technically it doesn't affect your device if new ones are better but the difference is so big it can be hard to resist.
3) The Internet! Web sites are getting more complicated, new services come and go, old ones change or have new demands for access as time goes by. Ultimately this is another young tech that's changing quickly and it's entirely possible that, say, all tablets that are on sale right now will be unable to access some web sites in a few years time because of performance issues, a new development standard, a new plugin (god no!) etc etc.