Compelling reason why I had to upgrade my iPhone 3G, iPhone 4 and original iPad after less than 3 years? Painfully slow and unusable.
Well if the compelling reason to upgrade is "painfully slow and unusable", then maybe a >= 5 year old PC that still works isn't a bad thing. If I'm not mistaken, my 2011 MBP's quad core i7 can still outperform the CPUs in today's Macbook Airs (the most popular Mac notebooks) in terms of pure processing power.