Analogy wise it is like Twice = snsd, Itzy = F(x), nmixx = Red velvet. F(x) only gotten its glory moment briefly before red velvel took over.
I don't think this is a good comparison. SNSD were huge yes, but FX had problems with their music and their popularity.
The Europop style was not catered well by Kfans as much, it only caught on recently. Most of their comebacks had terrible reception. Not to mention they got little promotions (unlike ITZY) and had problems with hiatuses..etc. SM didn't give FX a fandom name for years and years, and RV got it first.
ITZY were well promoted by JYP.
NMIXX too don't have the popularity and fanbases compared to ITZY's more popular members except maybe Sullyoon. That wasn't the case with RV, from the minute they debuted, Irene and Seulgi had fanbases bigger than expected, and the entire group had eclipsed Amber and Luna in popularity individually.
Also then both Krystal and Sulli had backlash over their "behavior" and Victoria was in China, so their fanbases were shrinking/taking a hit whereas RV's drastically grew with ICC.
I guess the Twice/SNSD comparison works, but I wouldn't put ITZY as an fx case, and NMIXX as an RV case. It's a very different situation.
Now back to the prompt.
Why not AJAX? They were DSP's first BG, had some buzz but their debut + lackluster promotions destroyed any buzz they had.
For groups that actually got out of...nugu: BAP, VIXX?, 2am,
Lovelyz easily.
SPICA? Competing with AOA then dropped.
Also MOMOLAND, Nflying, B1A4 (they used to be EVERYWHERE), Monsta X got left behind internationally but are doing better in Korea.
Pentagon..their fandom is doing better but it hasn't grown. CNBlue, FTISLAND too
TEEN TOP!
Also soloists are way more common nowaways than groups (in terms of falling off). Sunmi hasn't been doing so hot, Chungha has lost popularity, Hyuna is Hyuna-ing.
Also Hello Venus, only known for their change into the sexier side (not for the better) after their popularity dropped. DalShabet too, Dreamcatcher is doing so much better in comparison