htc one battery detoriation and how can mophie juice pack help??

how soon do you think htc one's battery will start detoriating?? if it does, will htc replace it for me?? because even experts find it difficult to repair.. ifixit gave it a score of 1/10. please help. this is the only thing that is holding me from going for the one...

Edit: and another question guys... What does the Mophie Juice pack really do?? does it act as a portable charger or as a secondary battery... i assume both are different things..