I loved my S4 but I was living on a boat for a year and it corroded just as if I had dropped it into the toilet. Now I have an S6.
Not sure about the updates, could be they stopped updating the older model just as Apple is doing with the IOS 11 release where it won't support 32 bit phones.
As far as the SD card, I guess it depends the app. Maybe if your phone fills it wants to be able to store data on the SD card if it is installed. One of the nice things I noticed after a recent update (could have happened earlier) is that I have complete control over each setting for every non-builtin app. At least it gives me the illusion that I control my phone :W.
Dave