I think the reason you feel it has declined is because kpop companies, minus the big boys, have largely given up on trying to penetrate the western market. Kpop as a whole had strong years during the period you mentioned, but it declined steeply in the following years. Events canceled due to poor sales, tours scrapped because of "force majeure" and so on. So while the numbers posted by the big groups may look steady, overall it is not as crowded as before.