In the old days, companies ended groups at will.
Sechs Kies was ended in one day. I think H.O.T. was allowed to have a goodbye concert, but it was lucky. Most acts simply ended when their companies told them to get lost, with the members thrown to the sea to sink or swim on their abilities.
With the TVXQ mess, things got a bit more sophisticated. Acts were put into long hiatuses, with fewer and fewer promotions, and on the year before the contract ends it was normal to switch mostly towards individual activities so some members who might be viable would have a chance to shine before the group ends.
Most 2nd Gen groups ended that way. It is interesting that Shinee still continues, but it is basically the only one remaining that way.
SNSD was phased out by SM, 2NE1 was disbanded by YG, etc. Sometimes companies do retain the group name and keep one or two members in the company , but they are disbanded for all practical purposes.
It might be because Shinee still has a foreign following which it retained despite of the passing of Jonghyun, while other acts did not retain their foreign followings after some members left the company, etc.
3rd Gen acts are still retained by companies because they bring huge revenues from outside of the country, but how many of them are remaining? BTS, BlackPink, Twice, Seventeen,Red Velvet, EXO, and a few others. All from major companies with huge followings. (There are examples like DreamCatcher but it itself is a reincarnation of Minx)
It is just that at the end of 2nd Gen, the companies decided it is not profitable to continue these acts anymore and ended it. It's all business, nothing more, and nothing less.
(A discussion of the 2nd Gen is not complete with the last remaining major 2nd Gen figure, who became the only person in the 2nd Gen to have a full 2 day concert in the Olympic Stadium in Seoul. Kakao did not get rid of her since she was a soloist and since she was not performance oriented she had much less costs, but Kakao did say goodbye to her, making her the owner of her own company. It only repurchased her company to make the size of Kakao Entertainment bigger. In other words, she only continued because she got to be her own boss, and being a soloist, had no members to please to make them remain in the company.)
