All of Starship groups sell above 100k so even if they only debuted Leeseo, Gaeul, Liz and Rei their 100k sales at least would be secured at some point.
About charts, wrong, their songs didnt immediately debut at top10 on melon, it climbed on charts cause their music is good. Even if Wonyoung and Yujin weren't in the group, they would still live up to Sistar's legacy because their music is good and different from what other ggs are releasing.
++++ To add, if Wonyoung and Yujin weren't in IVE Leeseo would secure her spot as most popular member in SK for getting viral, but it wouldn't even be like Billlie where only Tsuki is well known because Rei (most popular in Japan) Liz (most popular in China) and Gaeul are popular too.