I think the main problem is that they would quickly become a crude kinda exhibition fight division. In many ways when Fighters choose to retire, they dont want to fight again...If they do, its because they never wanted to retire in the first place regardless of what they say, and you do see that happen more and more often with long standing fighters.
Secondly...its kinda like asking a Lion, to become a Cat. These guys dont want hand outs because of their age, they are used to fighting the best, at the highest calibre. Not fighting as a sub-par version of themselves, against sub-par versions of other greats...you cant shrink them, for the sake of keeping them around. If there is one thing my academic work into the Violence Seeking Impulse revieled, it was that the only way is bigger, faster and better...you can not go backwards, and you can not return. It wouldnt feel the same, and they wouldnt get out of it, what they need in order to be able to perform.
But, whilst one can be a Champion whilst they are active...one can only really become a Legend when their time is over, because it is the passage of time, and the ability to be able to look back that solidifies their legacy. Their Legacy is not really their achievements, and their records, it is what they have given to the sport and how they have changed the history of certain organisations....Whilst the Accolades of a Champion can be compared to that of another...the same is not true with Legacies. They are independant, unique, stories of struggle and overcomming. Not every Champion becomes a Legend.
"A king has his reign, and then he dies. It's inevitable."