Let's look at a chart of each girl group's top charting song per year.
Rising:Who are the rising stars? As you can see on the chart Girl's Day and APink have seen steep rises in the past few years. Some might add AOA to this list but they have only charted in the Top 100 in 2014, it is way too early to tell if they will continue to rise. Similarly it will take a couple of comebacks before EXID solidifies their position as a rising group.
Falling:Also on the chart you can see several girl groups have fallen to the point of digital obscurity. Secret and Kara are prime examples as is F(x)'s steady fall. None, however, match the extreme fall from success T-ara experienced, going from #1 and #2 songs to #160 and then disappearing from the Top 200.
Other groups belonging in this category are Rainbow, Dal Shabet, Afterschool but they were never as successful as the others (with the exception of Afterschool during their first year... way back in 2009). They too, however, have gone from moderate success in 2011 and 2012 to not being able to chart.
Orange Caramel as a subunit is rising from #82 in 2012 with Bangkok City to #72 with Lipstick in 2012 to #31 with Catallena in 2014.
Maintaining:A few groups have managed to maintain their popularity. Sistar has maintained Top 10 songs with a slight dip last year to #19. While I Swear did not chart well, it was a follow up. Their next 2 comebacks will determine whether they are experiencing decline. In a similar, but more serious, boat is SNSD who saw a much steeper dip last year from the low teens to #43. Their most recent SNSD-TTS sub-unit song also under-preformed and their unprompted song Catch Me If You Can barely charted for 4 weeks. Their next couple of comeback will also be vital. 2NE1 is also in this category after a stellar comeback from a mediocre 2013. With a serious scandal their position is also precarious. Both SNSD and 2NE1 have to prove their position while Sistar only needs a Top 30 song.
Inconsistent:A few groups are very inconsistent, going from top hits to barely charting. On the chart 4Minute is the prime example with What's Your Name charting high but otherwise not ranking in the Top 50. Their new song Crazy may prove to be another hit but their charting is very dependent on their song, more so than other groups with larger fanbases.
Miss A is another example. In 2011 they were #3 with Goodbye Baby and in 2012, #12 with Touch but n 2013 only #96 with Hush. Hush would have charted higher if it had time to sell but not above #50. I Don't Need A Man also didn't perform well, however, Only You should make the Top 20 this year.
How about physical numbers?
*Note that SNSD's 2012 comes from SNSD TTS's album and T-ara 2013 number comes from T-ata N4's album.
Here it is more simple with sales either decreasing or increasing.
Decreasing:Within decreasing there are two groups; those who are still maintaining respectable numbers for a girl group and those who are not.
SNSD, 2NE1, Kara and T-ara are all decreasing but still maintaining decent numbers for a girl group. SNSD is still above and beyond everyone else while 2NE1 is maintains sales well above 50,000. Kara and T-ara, however, fell below 30,000 in 2014 and risk falling into mediocre sales.
Secret and 4Minute have fallen to mediocre levels although Secret was never much of a physical seller and 4Minute's newest album is doing fairly well. Miss A's sales have also fallen steadily by about 10,000 per year from 31,000 in 2011 to 21,000 in 2012 and 11,000 in 2013. Their newest album has maintained the 11,000 level.
Increasing:Girl's Day and APink have been increasing their album sales although APink much more so. F(x) has also been increasing their sales, become a top tier physical seller.
Maintaining?Sistar is the only group more or less maintain sales around 20,000 with 2014 being an exception.
So what conclusions can we draw?
In this group only Sistar is maintain numbers without any concern or speculation, however they are maintaining low physical sales and concert attendance meaning that even with a dip in popularity 2NE1 and SNSD are still at or above Sistar's level in many areas.
These groups are both increasing sales although they have yet to prove consistency. Their rise means they could be a temporary bubble. APink's strong fanbase, however, suggests they will be around for a while.
F(x) is on hiatus and their digital sales have fallen to the point that they are no longer as popular with the general public as they once were. With a strong fanbase they could make a comeback. Miss A and 4Minute really need good songs to maintain their positions but both had great comebacks in 2015. With good songs and a bigger fanbase Orange Caramel stands a chance to wedge a place for themselves in the girl group war.
Once popular with hit songs and for Kara and T-ara great physical sales, these groups are on the decline with Kara's comeback and T-ara's solo efforts barely charting. Secret's solo projects are doing better but that might mean the group will disband.
Obscurity Awaits:Nine Muses
Unlike perpetual nugus these groups have at least made blips on the Korean charts but they are seeing decline. Rainbow and Dal Shabet's 2015 comebacks flopped as did Hello Venus' post-Pledis songs and Afterschool is in a dungeon. Nine Muses shows the most potential but their company and constant member changes makes it difficult to be confidant.
I honestly don't know...Brown Eyed Girls
these groups have also appeared high on the chart (esp BEG) but BEG hasn't promoted in forever and their last few unprompted songs didn't chart that well. Mamamoo is in a weird ground between idol and non-idol and their latest song hasn't been doing well. Ladies Code had all the potential in the world... *sigh* I can't even talk about this...
Groups to look out for:AOA
All show potential to become Popular or National level someday!