In my post above I mentioned Gangnam Style by PSY. It should be noted that his personal take from sales, appearances, social media, live performances, etc. from that song alone was about $30M. That's obviously not typical, but yeah, the guy had a "one hit wonder" in his mid-30s but was then set for life financially if he wished to be. I think that sometimes happens with other bands and artists - They'll get in, crank out a good amount of hits, do some concerts and touring and then they don't really need to work after that if they don't wish to, or they're free to pursue other projects that are meaningful to them, or they may be interested in working in other areas of the business - writing, producing, etc,
There are a few smaller venues (casinos) near me that feature concerts by artists who peaked in the 80s and 90s, including some rock groups from back in the day. While some of those groups and singers may still be touring and performing because they need the money or have other reasons they must continue, I think others do it simply because they still enjoy it. They enjoy performing, they like the rush they get from working in front of a live audience, they get a kick out of the fact that people still line up for their autographs and as long as it's only maybe a few dozen dates or less per year, it's not a real burden on them, their families or their time. The set lists and choreography are most likely the same as they've used for years, so rehearsals most likely aren't needed often.