Without any empirical evidence, I'd say you could easily get at least 5-6 years out of any computer, maybe even more.
Apple has done a lot to remove moving parts from their computers. SSDs are more reliable than spinning platter hard drives as they have no moving parts. The battery will eventually lose it's capacity, but they are rated for 1000 cycles. That doesn't mean that the battery will be useless after 1000 cycles, just that they expect about 80% of the original design capacity. The fans have bearings which will eventually fail, but you're probably looking at 6+ years of average, day-to-day usage (as opposed to a server/ datacenter that runs 24/7). The new MacBook has no fan so no moving parts!
You also have to take into account that components just fail. There's no rhyme or reason to it, and there's usually no prediction that a component is in failure. It could just as easily be 13 months after purchase as it could 72 months after purchase.
Finally, there's software and "keeping up with the tech". If you aren't a compulsive upgrader and you don't have to keep up with the latest software (and stick with what works) then there's no reason that in 2018 the machine won't work as good as it does today (hardware problems notwithstanding).
