Is it that iPads are not upgradeable? Well, Apple has an entire system for recycling and credit. Also, an argument could be made that an upgradeable iPad would also cease to be as useful as an iPad, as it would necessitate some level of form-factor change.
Is it that software keeps moving forward? Well, that is not new, and is not unique to an iPad. A five year old computer running a recent browser will also struggle with new websites.
Is it that the author can't give the iPad to someone else who could use it and wouldn't be as frustrated? I fail to see how that is the case.
I'm sure using a five year old computer can get frustrating. But I think the fact that an iPad is still quite functional after five years of use is rather remarkable. No one writes op-eds in the NYT lamenting the slowness of their five year old netbook, because those devices are even more disposable than an iPad.
Edit - that is not to say that there is not an issue with aging iOS hardware. I just don't think the article makes a clear argument, or even statement, of what that issue is.