The SP3 is designed like any other laptop. It definitely may get warm under a heavy workload, but that doesn't take years off of its life. If you take care of it, no reason at all that you couldn't get at least 4 or 5 years out of it. And even then, if it was running well, the only reason you'd upgrade would be because you wanted something more modern.
I have a few folks in my company that routinely make their laptops last 5 - 7 years. Even the laptop that I had before the SP3, a 12" with a 1st gen i7, is 4 years old now but still works perfectly. In fact I had to do some major begging to be able to buy the SP3, because my 12" laptop is still perfectly fine.
That being said, if you do get an SP3, then you'll definitely want to spring for the $149 Microsoft Complete warranty.