How much time does the new Mac Pro (late 2013) lasts as an usable computer?

So, recently I've been using a base model iMac from mid 2007 as my daily computer. Sure, it's slow and it even crashes when I open too many tabs on Safari but, still, it's a perfectly usable computer for the average user.

As we are in 2014, the computer will turn 7 in a few months and it let me wondering: how many years does a computer hold out as an usable one for the average user?

Take the top notch new Mac Pro (late 2013), for instance: a $9,600 machine, 12-core monster with PCIe-based flash storage and dual GPUs for up to three 4K displays, how many time does it hold out?

If my iMac, an average desktop with the lowest settings Apple offers (except for the RAM, wich was upgraded to 4GB), is 7 (supposing this is its last year), this new Mac Pro should last like what? 12 years from now? Please share your thoughts below.

My iMac specs:

Processor: 2GHz Intel Core 2 Duo;

Memory: 4 GB 667 MHz DDR2 SDRAM;

Graphics: ATI Radeon HD 2400 XT 128 MB;

Software: OS X 10.9.2;

Display: 20-inch (1680 x 1050);

Storage: 250 GB SATA Disk.

I'm sorry for any grammar mistake, I'm from Brazil therefore english isn't my native language.