Well established and successful groups often still bring in enough money to keep them going even if the "hype" is over. Instead it's often the members that want to do other things that kills the group or other problems and scandals. For example Girls' Generation, even if the hype is over a couple of years ago they still probably would earn enough money to make the company happy if they wanted to be active, but instead it's the members that want to do other things in life.
2NE1 was a combination of problems and members that wanted do solo stuff that killed the group.
But lesser popularity and failure to generate "hype" of cource also play in when members and companies decide if they will go on or not. Like T-ara that more or less crashed 2013 and forward from being a tier 1 group during 2011-2012. I can understand if members lose their motivation to go on if you are less popular (= earn less money) than you where a couple of years ago.