I still use mine almost every single day. It's a beast, and has been very reliable. Keyboard is WAY nicer to type on than the current generation. It's a little thicker and heavier, and lacks USB-c ports, but it's still damn fine.
Super reliable machine, still relevant performance-wise.
I plan on using it until it disintegrates.
Edit: I wonder if apple regret making these machines. They seem like a failure of planned obsolescence - at least, until new releases of OS X stop supporting them, which will probably be the next release, looking at the current horizon. Anyway. Totally broke the upgrade cycle for me - I have grown rather fond of my (t)rusty steed. My mother upgrades more often than I do, usually because she’s left every process open and running, and the machine is slow, and therefore she thinks she needs a new one.
It's a good point about the planned obsolescence: I've dropped this machine so many times it doesn't quite close properly, the aluminium around the top of both USB ports and the HDMI port are deformed, I've had to clean metal particles out of the MagSafe2 connector many times, I work in a metal fabrication workshop so metal particles follow me everywhere.
Six year old machine still in reasonably good condition and works fine.
If your old products are still running fine that's only good publicity and generates word-of-mouth credibility.
But second-order (and beyond), particularly from the perspective of the companies, I'm less sure what's best, or if there even is a global, objective 'best'.
Consider the most radically distant point you can imagine in 'self-supporting human group' configuration space, that's still capable of making something like a MacBook. What does it look like? How is funded/financed/supported? Is it temporary, i.e. more like a one-time project like making a movie or a video game? Or is it, somehow, a 'going concern'?
The 'going concern' attribute is one that I think is perhaps under-appreciated. One thing that companies, and other long-lived organizations, do well is in organizing people with specific expertise long-term. Think about making a computer, or a washer, or any physical product. Surely the people that make those things, that have done so for an extended period, have a huge amount of detailed knowledge, and wisdom, pertaining to making the specific things they make. What would be lost by them NOT having made the same things for so long? What would our computers, or washers, or pencils, be like if they were all produced by something more like a Kickstarter project?
Imagine another possible world where the 2013 MacBooks were just as good as in our world, but also intended to last indefinitely, perhaps more easily repairable, or made of more common or standard components somehow. Say they lasted 30 years, easily, before needing to be replaced. How is Apple different in this other world? How do they retain engineers like they do now? How do Apple's competitors do this? And, maybe most importantly, how much do MacBooks cost in this other world?