How good songs do in charts will often show how the popularity of a group is falling or rising:
If a group is very popular, their songs will draw more interest and if a song becomes a hit song, it will boost that group's popularity.
During their career, groups will experience peaks and dips in their popularity, until there's no more peak to speak of and a gradual decline sets in.
Based on that, when was each group's last peak? This is what Gaon shows:
2015
Girls Generation, 9th year of their career (#44 on year chart)
BIGBANG, 10th year of their career (#1 on year chart)
EXID, 4th year of their career (#28)
2016
Wonder Girls, 10th year (#22)
Sistar, 7th year (#44)
2017
EXO, 6th year (#14)
Winner, 3rd year (#10)
2018
iKON, 3rd year (#1)
GFriend, 4th year (#34)
AOA, 6th year (#78)
2019
ITZY, 1st year (#11)
TWICE, 4th year (#29)
2020
Red Velvet, 6th year (#9)
Mamamoo, 6th year (#29)
APink, 10th year (#56)
2021 (still peaking)
Brave Girls, 6th year? 11th year? (#2)
Blackpink, 6th year (#14)
Oh My Girl, 6th year (#15)
Boy groups are harder to track because they hardly show up on Gaon year chart. Also keep in mind that if a group gets a new superhit song they can have a new peak in future.
Look at Brave Girls and Oh My Girl, how they have their best peak and even their breakthrough after many years in their career. In the case of Brave Girls even on the verge of disbanding.
