Maybe not a decline but a correction. Like how stocks are overvalued and it trends so high that none wants to buy in, then the market correct itself and the price is affordable. Maybe KPOP is doing the same thing?
All the hyped lasted for a while and now those people doesn't care anymore and so we see volume decrease, leaving real fans, maybe?
KPOP shouldn't be in your face "AHA look at this new thing here!", but rather it should be a genre of music that ones actually listened and enjoyed, it's healthier for the industry.