You are basically arguing "for many people an iPad is a better computing device than a PC because there is less that can go wrong with it". Which I think is a very valid point, however it does not hinge on how old these PCs are - your argument works just as well for new PCs without properly configured backups and anti-virus protection.
If you read Phil Schiller's statement in this way - "it is sad that many people are using PCs which were better off using iPads" - then one might still disagree, but it is not offensive, especially coming from an Apple executive. :)
What the author of the article finds offensive - and which also put me off when watching the live presentation - was the suggestion that using old hardware is "sad". When you read Phil Schiller's statement as "it is sad that many people are still using five-year-old hardware" (which was my first interpretation as well) it really reads like advocacy for blind throw-away consumerism. Which also makes a mockery out of the picture Apple was trying to paint just 30 minutes earlier during the same presentation: That Apple products are built to last so that you do not have to throw them away every other year to replace them with something new because they stop receiving upgrades, their hardware fails, etc. (Which, unlike the author, my experience by the way supports - my old iPhone, old iMac and old iPad still work flawlessly, especially when I compare them to the number of Android / PC devices my friends have already rotated through in the same time period.)