I agree with half of it. Her death was pretty ham fisted and inelegant and smelled, to me, like they had no idea what else to do with her. Or Doran. Or Trystane. Or Areo Hotah. Or that whole section of the continent. Cersei was like "oh... huh... just like that witch said. I guess she was right." It was more about her holding on to Tommen and Jaime than seeking revenge for Myrcella or any of that. I bet her vs. the sand snakes gets dropped for awhile (watch Sunday be all about the fucking sand snakes) until another element gets added to it. Maybe a small, bearded, fugitive element.
I think it's crucial to Cersei's development. Compare Cersei's reaction to Joffrey's death vs. Myrcella's. In Joffrey's, she screamed bloody murder and demanded Tyrion's head. In Myrcella's, she's just defeated and surrendered to the witch's prophecy. This last episode with her bit about Varys's birds brought some of the old Cersei back, but right now, the Lannisters are looking completely powerless aside from having The Mountain on hand. Their comeuppance for a lifetime of treachery looks like it's coming to a head, even without the Starks doing anything to them at all. It could be at the hands of the Sand Snakes, or really anyone at this point in King's Landing's rotten state.
Just for conversational purposes, it seems like faith and magic are taking a front seat this season, compared to others where it was lightly touched upon. Jon is back from the dead (with heavy emphasis on Lady Melisandre's struggle with her faith), Cersei feels beholden to the Witch's prophecy, Bran's warg flashbacks take up significant screen time, red priests in Meereen (and Volantis last season) are praising Daenarys like Melisandre latched onto Stannis and now Jon Snow, Arya is magically going blind and back...
On that note,
every trailer for this season has teased at every house falling and everyone dying. I've only read the first book so I'm not familiar with the Faceless Men's methods outside of what the show has shown, so let me know if I'm stupid, but part of me thinks Arya, or her earthly form at least, just died herself. Whenever any other volunteers drank the water, they did so looking for relief. Arya drank the water and experienced relief, which might have been what happened from the drinkers' perspective, while somebody watching Arya might have just seen her collapse. The House of Black and White, for being a worldwide network of hitmen, seems to be only be composed of Jaqen H'gar, the shitty bitch Arya trains with, and maybe another servant here or there, all of whom interchange faces or disappear in thin air, so it seems like they're not among the living.
So far this season has me gripped. It's really somber and dark. I'm braced for everything to go to hell.