Elvis's apparently petulant refusal to release new music in the wake of 'National Ransom' has, in my opinion, contributed to his failure to ascend to the status that Bob Dylan managed to secure - that of being cool and interesting to new audiences late in his career. Elvis is an elder statesman of sorts, but one whose work is much more 'respected' than actually listened to. The lack of a 'third act' to his career isn't helping whatever claim to enduring significance his career may have.
On this wider question of legacy, Dylan is again the gold standard: we're living through the first phases of the struggle over Bob's place in the canon of western art, and the early returns are very promising, what with Christopher Ricks throwing his considerable prestige behind the cause and the Nobel Prize putting the official seal of approval upon it. Academics hold conferences of his oeuvre. There is a decent chance that Bob Dylan will be an artist whose work is regarded as significant for generations to come. The Beatles are in a similar situation, although here the groundswell is perhaps more 'popular' than upheld by the official gatekeepers; those songs have just become part of the air we all breathe, and will be sung around campfires and as lullabies, as well as being listened to as fascinating recorded artifacts, for as long as our culture endures - or so I believe.
Below those two are artists like Leonard Cohen and Joni Mitchell. They may or may not enter 'the canon' at some point, regarded as authors of a body of work that repays serious attention. Failing that, these artists leave some songs that are guaranteed to live on; Cohen, especially, in sufficient numbers that he is unlikely to be forgotten. It helps that he supplements his music with some pretty good written poetry.
I think EC's legacy stands to be on the tier under this. If I had to guess, I'd say that two or three of his songs may survive into the indefinite future. Allison, Almost Blue, and Shipbuilding would be my candidates. The rest is apt to recede into a semi-forgotten museum piece of interest only to the occasional curious person who might want to find out more about this mysterious character who wrote those songs. Ironically, perhaps, The Juliet Letters may outlive his much bulkier 'pop/rock' work, being attractive to the rarified and small circles interested in quartet and voice. My guess is therefore that EC - neither popular enough to have his songs enter the general lexicon, nor genius enough to enter the realms of canonicity - will be *largely* forgotten once the generation represented by this message board evaporates.
When man has destroyed what he thinks he owns
I hope no living thing cries over his bones