There are two ways to look at his question: -
1) How did Elvis benefit from the Colonel?
In the beginning the Colonel was the man who directed Elvis' career and got him the million dollar contracts! The Colonel of course benefited also. When Elvis was in the army the Colonel kept the home fires burning and Elvis came back to a decade of good to poor movies (I'm grateful 33 movies are committed to celluloid), that said the money kept rolling in.
Had Elvis gone back on the road would he have lasted?
I think we should also point out that if Elvis wanted to get away from the Colonel the mid 60's would have been the best opportunity, by 1968 when the 68 Special was aired Elvis was big news again and there would be no way the Colonel would every get rid of the Golden Goose.
Elvis and the Colonels relationship was very intricate, they both needed each other to get what they each wanted. I personally believe that Elvis had become resigned to the fact the Colonel knew way to much about Elvis personal life, requirements and idiosyncrasies for someone else to take over.
In the end it was pure apathy that Elvis didn't get rid of the Colonel, he was too ill and the prescribed meds didn't help.
Lets be open and honest here, the Colonel gets a lot of stick (same as Priscilla, rightly or wrongly) The Colonel had his people around him and so did Elvis. Elvis lived his life as he saw fit, is it really the Colonels place to intervene? Elvis and the Colonel were business associates, they weren't close buddies. The Colonel did do his job to a certain degree, nobody knew about Elvis' lifestyle until 1977, perhaps if it had of been sooner things could have changed.
2. Would a change of management have benefited Elvis?
This is a real tough question and would depend on when that change would have happened! Perhaps if Elvis would have changed in the mid 60's it could have been a good thing, a new direction? More serious roles in the movie business? Back on the road? Who knows what could have been.
I certainly don't think Elvis could have changed his management by 1972/3. Elvis had become too set in his ways and the Colonel also. I think the prescribed medication was an issue that would have had to have been dealt with. There are without a doubt missed opportunities here and there, movie offers, world tours etc. It takes two to tango. We Elvis fans are always going to stick up for the king, who knows if the Colonels had given up looking for new opportunities for Elvis given his medical health etc, perhaps a fresh perspective may have re-energised Elvis.
I don't believe the Colonel understood Elvis' addictions and so wasn't in a position to help, I'm sure he tried as those who were around Elvis tried. Sanatoriums weren't around at that time and would the colonel have committed Elvis even if there were, could a new manager have helped?
I think mistakes have been made on both sides, it is too easy for use to make snap decisions with the benefit of hindsight.
Given that Elvis is a legend and one of the biggest names in entertainment history, Elvis and the Colonels relationship for all intense and purposes was successful although tragic.