sometimes yeah, lip syncing is the only option for hard choreos, bc let's be honest - that level of multitasking is hard to do. But I also feel like those who lip sync aren't putting very much effort into their performance. A K-pop performance isn't just about dancing - it's about showcasing your vocals and rap skills, too! You have to have the whole package in K-pop imo.
Now, if a member was sick, that would be a different story. We wouldn't want Jungmo or Nayeon to have to strain their vocal cords bc they were sick during a performance - that would be cruel in my book!