Sure, I get that. It just seems that most of the library of apps will still be available for older idevices even many years later. Think of all the apps that were written before the newest iPad was released. There are tens of thousands of them. And, you can always not take updates if they require certain hardware specs.
I'm not app crazy, so I really only care about whether the apps I use are affected. I figure individual owners are best positioned to decide when their iPad needs upgrading, depending on how they use it. Your usage probably varies from mine, for instance.
For me, crashes and sluggishness are a big strike, because I use my iPads for surfing more than anything else -- that's how I read nearly all of my news (I read about a dozen news sites every day, and read links off them as well, for instance, and I speed read). The little work I can do on iPad also is also limited by crashes and sluggishness.
The other iPad apps I use most frequently (for books, audiobooks, movies and email) tend to have more backward compatibility than say game apps, which don't interest me at all. With apps I use on the go (like travel or search-specialized apps like Yelp), my phone is the key thing most of the time, and I'm using an Android phablet for that.