with Elvis it wasn't really a "decline" but more of "not caring anymore",he still had it the last couple years but it all depended on his mood .when he didn't care his performances suffered (Aka:June 19th 1977), when he did care his performances were still great (Aka:Dec 31st 1976,June 26th 1977),the same with his studio stuff,he stopped caring in 1971 with the christmas album and you can tell,then after that it all depended on the song,some songs he cared for and you can hear it because he put more effort into them while others he didn't care for and he didn't put as much effort into those songs,but he still had the voice and stage presence,even though I do agree that after 1975 it became VERY obvious,especially with the way he looked and perform on stage..
As far as his voice was concerned, I don't think Elvis ever 'lost it', he just got really bored of singing the same old songs on tour. He was generally far more into the 'big voice' ballads like 'It's Now or Never', 'Hurt' and gospel classics like 'How Great Thou Art'.
From what I've read, sadly, I think Elvis got scared of a lot of things - scared of getting older, death threats, poor business decisions (Graceland was indeed in the red and partially mortgaged out at the time of his death) his deteriorating appearance and some genuine health problems (ie clinical depression, possibly undiagnosed bi-polar I speculate, glaucoma, as well as a distended colon, which was very painful and caused some unpleasant complications from what I've read) though IMO much of the personal deterioration was caused I would say by his unhealthy lifestyle), scared of being lonely, not knowing who and what was real. And obviously spending too much money. He did work VERY hard from 1969 to 1977, over 1000 concerts, I believe.
Obviously he thought the industrial quantities of prescription drugs he was consuming were 'medicine' (like some southern US folk used to do, I believe), but of course, they were destroying his body, his reason and making him paranoid. He fired two of his guys in '76 and the next year, they wrote and published an expose on his drug abuse etc. Elvis died the day before he was due to go on tour for the first time since that book was published. He was terrified of people in the audience shouting 'Junkie' at him etc. On some level, he must have known he was seriously out of control with the drugs and diet, but he couldn't stop and nobody could make him ...
He really should have taken at least a couple of years out from showbusiness from '75 or '76 to deal with his health and drug issues, rest up and get back in condition before coming back in say, '78 with a more modern look and style and some great new songs.
On a deeper level, I suppose you could say he just got sick of being 'Elvis Presley, Handsome Superstar' and knew the reality would never again match the myth. Also there's some truth I think, in that he never got over the loss of his mother and the blow to his pride of his wife leaving him for another man, despite all the affairs he himself was having. Reality can only be avoided for so long.
