There are, however, instances when problems can occur right after the update. But those who really wish to update their smartphone to the latest Android OS can actually do so through custom ROMs based on Android 6.0 Marshmallow.
As to why the Galaxy S4 no longer received the Marshmallow update, smartphone manufacturers have set a two-year time frame for its devices to take on mobile operating system updates. While there is no rule that set everything in stone about this, the reason why the device makers have set an allowable period to update the OS of their devices is to ensure that the smartphones can still perform reasonably well.
As everyone knows, every OS update usually gets bigger and bigger, as far as file sizes are concerned and an older smartphone can only take so much before it practically pukes and can no longer handle it.
Apparently, Samsung has a good basis for no longer allowing Android 6.0 Marshmallow updates to the Galaxy S4.
It turns out that the device has been suffering from sub-par performance when it took in the official Android 5.0 Lollipop updates early last year that taking on further upgrade would have a telling impact on the performance, functionality and usability of the device.