Let's assume for a moment that I bought the latest greatest Mac Pro today and Apple and third-party software companies would support the hardware with the latest version of Mac OS or their software through 2017.
There is no way I would still be using that Mac Pro as my main system 7 years from now. Why? Because that Mac Pro is going to be a dog compared to what is out 7 years from now.
Moore's law is the reason.
S-
Moore's law doesn't apply to what is or is not useful to you. Just because in 7 years the average chip will hold 8-16X the transistors doesn't make a machine more or less useful, it just makes newer ones more extravagant.
I have a friend who is an author and makes...well, let's just say he's very, very comfortable. We writes his books on a Pismo. I asked him if he was ever going to upgrade and his response was "when it breaks"...