My theory is just the core fandom has just gotten older and at a different stages of life now where theyre not as free. They simply don't have time to do stuff like mass stream and their comeback is just not vibing with the GP.
That explains why sales numbers remain high and continues to grow, because those fans can still purchase albums even if they don't have time.
They're not also acquiring new and younger fans as quickly as other groups. 4th gen groups will naturally acquire new kpop fans just being more more new, recent and trendy. Just like now when kpop grew here, fans naturally check out 3rd gen and not 2nd get, we're now at the stage where new fans are checking out 4th gen instead.
Twice concept and image is not as domestically resilient as other concepts, the cute concept. Sadly Korea and alot of other similar countries focuses alot on youth. Alot of these fans will jump ship as soon as a newer, younger, fresher group debuts and they have been doing so for years slowly. But again, theyre still growing and gaining fans, that has not peaked but just not as quickly as others. Young members is a commonality amongst the top groups right now if you haven't noticed.
This is why some groups are shifting internationally as which I think is smart. The US specifically couldn't careless about the youth of a group. Twice international numbers are decent. Their global Spotify is doing just fine, touring numbers are phenomenal, US album sales are better than almost all groups, etc.
Twice is at the stage now where they can't just top charts with their name alone, but most of 3rd gen cant either. Their release have to actually be really good and TTT is great IMO but it's not 5 star great which is why it's not tanking like most group would by this point but still stable at the 30-40s. It's been 7-8 years already, en entire generation has shifted. Original Onces are now starting college or work life.
Of course there will be exceptions, so don't quote me going this group this and that.