I bought my first Mac Pro (1.1) new in 2006, with some upgrades it lasted some 11 years and even though it´s not officially supported by Apple, with El Capitan I would still be able to use it for some years.
I was able to buy a 5.1 for a reasonable price, so for now I´m up to date OS wise, and most likely settled for another 2 to 4 years I guess.
I was looking at buying a trashcan, but I didn´t feel like paying a premium price for a 4 year old machine…