"Over the next year or two, though, I'm sure there will be plenty of software that I'd like to use, but won't be able to."

FWIW, that's what I thought about my iPad1 when iPad2 came out, and again when New iPad came out. I'm perfectly happy to upgrade when needed (I bought mine early on when they launched here in Australia, so it's over 2 years old now, which is a fairly acceptable lifespan for tech gadgets for me), but so far I've still not bumped into any app I want to run but can't, at least not anything compelling enough to push me to spend money to upgrade. I suspect if I were more of a gamer, I'd be running into performance problems, but for all my use cases, iPad1 seems to be perfectly happy to run that apps I want - and short of some category-changing new app idea, I'm pretty sure all the apps I use will continue to work for quite some time yet, even if they get updated with new versions which require iOS6 which I'll miss out on.

I'm strongly suspecting now that the battery will need replacing before the device becomes unusable due to newer OS and/or software requirements.