Let's be honest. There are 3 things that all run out eventually:
Fans save their own money to help fund comebacks, whether it's to download from official stores, iTunes, or any other streaming services. But money is a finite resource. Once the savings run out, people have to save up again for the next release. They can't spend money forever. Prime example are the boy group/male idol stans I've seen mass downloading on Bugs and Genie, yet are nowhere on Melon or FLO. They can buy a lot in those first 2 weeks, then the funds run dry. The song drops like a fucking rock on Gaon Digital after peaking in the top 10 for a week or two.
After the first couple of weeks, only the fans are actively talking about an artist's new song, unless it's a viral hit. Most people move on. Only fans and the casual listeners who enjoy the song stick around to stream it.
Once money and hype are low, the fandom will slowly start to pull back from supporting the song in a hardcore fashion. Fans aren't motivated anymore once the song is losing public interest, if there ever was any to begin with. Especially in K-pop, once promotions are over and the music show performances wrap up, the song doesn't stick around in public consciousness.
BTS had that problem for a while, but once they started releasing their biggest hits (BS&T, Spring Day, DNA, FAKE LOVE, IDOL, BWL, Dynamite), they stayed around for a lot longer than is typical for an idol song (i.e. high for a minute, then it falls quickly over the following weeks). Most idol hits are lucky to get anywhere near a year on Gaon Digital Chart. Even ON hit 51 weeks, which is 1 less than FAKE LOVE.
1 year is a norm for BTS. Any single of theirs that doesn't hit that is an underperformance in my mind, which is different from other idol groups.
40 weeks or above in the top 100 is huge for any other group besides BTS.
If BTS's biggest hits were really fandom-driven, they'd peak high and then have 0 longevity. They'd sink like anchors and be out of the top 100 in less than 3 months, but they don't. There is public interest in them and it shows in how their songs chart.