Well, actually most of them choose to stop working in the industry. After years of strict diets, tons of rules, stress, exhausting schedule, it's natural for them to want a normal life, where they can actually...live. Most of them invested already in business by buying stocks or starting a small family business like opening a cafe, or they bought apartments and rent them to have a stable income monthly. Which are smart moves. The ones that complain about money after they hit the success and gained some money, shows that most probably spent their money in a stupid way, like on expensive cars or clothes.
But yeah, for those who want to remain in the industry is still hard. Some people can't shine individually, they need a team with them, that's the reasons some of them never receive solos or they aren't asked to act. Even if they have popularity, acting is not for everyone because it can ruin the entire production if their acting is fake. For solos, well, you would be surprised to see how many of them can't sing and always make playback.
For variety shows, if you don't have the power to talk properly and maintain a good atmosphere is pretty hard. They need good speaking skills, a good memory, and other things.
Some idols are relevant even after disbanding, they can be models or DJs or, if they have skills, they can produce/write songs.