I think in order to look at anything that has a history behind it-you must examine that history as a reference for the possibilities for that thing down the road.
I would give the odds of Elvis becoming a hasbeen-a 3% chance if he had lived...based upon the history I have already shown in other posts.
Fabian became a hasbeen, so did Bobby Rydell, Gene Pitney, The Flamingos, The Four Tops, and a thousand other hitmakers since 1956....Elvis was not a follower "he led" and leaders do not fall by the way side whether they are still leading or whether they are now more background- but still moving forward.
The legacy was always intact-every entertainer at some point is taken for granted, there career ebbs and people forget for a time the greatness-but its that greatness that springs back into action in some form that reminds people of how important those people have been and continue to be.
When Elvis died Geraldo Rivera said he had to convince ABC news to open with the story of Elvis' death-he told them you do not have a real understanding the importance of Elvis Presley life and career, to not just popular music but to international culture on all fronts.....and indeed the outpouring of public mourning and respect for Elvis' cultural significance was exactly as Rivera told ABC it would be.
After the 68 special the next day in school the "watercooler talk" was ELVIS and how great he sounded, how thin he was...the great leather suit etc.....ELVIS surprised everyone and each deep hole his career got into.....he surprised us. FACT
I am not just leaning toward how special he was compared to hundreds of other cultural figures from any decade-I am absolutely convinced.
So whether he died at 42 or lived to 100 for most thinking knowledgeable people his accomplishments and significance would always be seen.