Hmm for this instance i would like to agree with Dreamer. Yoona was not the most popular member of SNSD. She was known for being visual + center but Taeyeon has always been more loved (especially by their fanbase, alongside jessica)
Tzuyu totally overtaken by Nayeon & Sana after the flag incident.
In both cases, they are popular for being the visuals but they were never the super IT girl (Yoona maybe was during their earlier career but towards mid not so much i feel)
This is a total rewriting of history. Taeyeon was simply NOT as popular as Yoona until after she built up her solo career, and never had been. Jessica was also more popular than her until being kicked out. For how large of a role she played in their music, Taeyeon was standing in the background up until that point, which is a big part of why people point towards her being responsible for Jessica's departure.
Tzuyu was absolutely an "it girl." This isn't debatable. Her visual could be seen all over Seoul. She was getting solo endorsement deals when no one else in Twice was. This didn't change until JYPE sabotaged her career following the flag incident. But to this day Tzuyu hasn't been "totally overtaken" by anyone in Twice (which is probaby part of their problem.) She still has the 2nd highest number of followers on Instagram for instance, behind only Momo. She just doesn't get artificially pushed into the limelight like some others *coughnayeoncough* for entirely political reasons.