My S4 is still going strong, and it has a removable battery. It also heats up badly, but so far so good still.
The problem with unibody handsets, is that if a leakage does occur, it will run onto the board, and that accompanied by heat, any form of heat can cause serious damage.
I have to say something, while people generally put their phone in the pocket, its not ideal. There is sometimes no room for movement, and peoples body heat would cause moisture, and any moisture seeping into the phone, coupled by heat, could cause problems. Same applies to women who put their phone in their boobs

, dont do that, you could get hurt.
I also want to say, dont use the phone while its charging, or over charge it. If it says fully charged, unplug it.