You are right, and it doesn't really matter what a popular person does - his followers will look at it with interest anyway. If you take twitch, then the most popular streamers, no matter what they do - from just chatting to playing some kind of stupid game (not interesting to anyone if you don’t take into account who plays it) always collect a huge number of viewers. Similarly with these fights - legends are always interesting, even if they have grown old and cannot be competitive in purely professional sports.
As long as the promoters do their job it can be converted to a huge amount of profits, legends are happy for giving them chance/opportunities to step back inside the ring, with a good amount of offers and with some medical securities that will be discussed behind those fighters even they are no longer on their fighting shape they will accept the challenge.
There's something inside them that will continue to be delighted once a fight being schedule, especially if they see fans who are still cheering for them and still happy to see them fighting.