Yeah, one problem all these shows run into is the need to attract as many viewers as they can, like you said, so naturally they go with some bigger name actor/actress. Then, they get hamstrung into shoving that actor/actress into every episode whether they should be there or not.
Game of Thrones had this problem with Lena Headey. Her story should have been fairly minimal after Dany burninates all her armies and the Night King comes south of the Wall. Instead, the idiots running that show made the NK a joke and wrapped that much more impactful and interesting storyline as quick as possible so we could get back to King's Landing and Cersei's bullshit. Like every episode you have to shove Lena Headey drinking wine at a window into the episode.
WoT is suffering from this with Moiraine because, well, you paid Rosamund Pike and she's the only even somewhat famous name on the show so you're getting Rosamund Pike. It seriously makes me wonder what they're going to do when it comes time to have