The battery in your phone is no longer holding a useful amount of energy to power the phone all day. You could replace the battery and bring the whole phone back to "like-new" condition, but the manufacturer has glued the case together. This is the malicious intent: The manufacturer *KNEW* that the battery would not last forever, and still welded the case shut. You would not stand for this if it were your car.
I really don't like defending cell phone companies, but I have to play devil's advocate on this one. Gluing all the components together may be the only, or most cost effective, way of giving the phone enough structural integrity to not bend when put in your pocket. These things are