The sad and unfortunate truth with technology is that some day it's going to fizzle out and be surpassed by the next best thing. This is just the way of things in the technological industry as things move extremely fast. This also applies to software updates for devices like smartphones, and in this regard most devices won't have software upgrades for more than a two year period. The Samsung Galaxy S III for example is well past its software update lifetime, yet the international model has been lucky enough to have been graced with the majesty of Google's latest Android software version, Android 6.0 Marshmallow.
This is amazing in it's own right given the scope of time it's been since the Galaxy S III was a current device. Three generations back now, almost four as Samsung are likely preparing for a launch of the Galaxy S7 early next year. It's worth noting though that this is not a direct software push from Google and Samsung, but rather the first nightly build of CyanogenMod 13 thanks to some crafty developers. Whether or not this is going to be an ongoing series of nightly builds is still unclear, and how long they'll last is anyone's guess, but for anyone who still owns and uses the international model of this device which was perhaps the most popular Android smartphone from Samsung to date, any version of Marshmallow is better than none as it gives the device a chance to freshen up its user experience.
Those who are thinking about trying this nightly build should be aware that since it is a nightly, things are going to be broken. What's interesting is that there aren't too many known issues that are listed, which is something that might have been expected with such an old device with older hardware specs. According to the XDA Developers thread, the only issues are that WiFi turns off occasionally after the screen goes to sleep, WiFi p2p is non-working, non-FAT 32 SD cards are non-working, and there are some issues with the colors of video in Netflix after watching for a prolonged period of time. If you're running this model of the phone and are interested in Marshmallow, you can find the full instructions on the XDA thread. Those who aren't familiar with the process of installing ROMs are likely better off leaving this alone though.