the VERY FIRST iPad (although you said iPod) will not take iOS 7 either. And that’s because Apple is FIVE GENERATIONS into future iPad models right now, which are much more powerful and have more RAM and run systems that require more computing power and have apps that require more power.
The reason why customers buy NEW MODELS “today” is for that “more power” and more capability and more advanced apps.
You would have the ENTIRE MARKET held back, even FIVE GENERATIONS later, just so that all the apps and the system can run just as slow as it originally did with no additional power to it ... LOL ...
And at the very least, the publishers could keep their previous iOS5 and 6 versions also available on the app store for users of the older devices. But Apple has an obvious incentive not to allow that, and they don't.