And what is this 'plot' you are talking about?
I was of the impression, that there never has been something like an overarching plot in this series. ASoIaF, but also the TV series Game of Thrones, both tell the many different stories of dozens of individual characters and maybe someday, all these different plot lines will merge into something bigger, but maybe not. It certainly wouldn't surprise me, if some POV characters and their storylines don't have any impact on the outcome; but that doesn't mean that they have no reason or right to exist.
In the books, but especially in AFfC, we spend so much time in the Riverlands, basically all of Brienne's and Jaime's stuff takes place there. You don't really believe that all of this is irrelevant, and instead, the freaking Sand Snakes doing basically nothing for an entire season is more 'important' and thus, had to be included at all cost.
David and Dan are competent writers, but the changes have completely gotten out of hand this season. They have turned the show into something that very much resembles fanfiction. I still do enjoy the show for the most part, because the actors are great and the visuals and the music are outstanding. Also every now and then, they do something grand and epic, like Hardhome - but honestly, I'd prefer a more faithful adaptation of my favourite series of books. But that's not what Game of Thrones is, not anymore, and that's kinda sad, in my opinion.
Lady Stoneheart is just one of many amazing characters and storylines we'll never see on the show. Personally I think it is very unfortunate and kinda disappointing, that David and Dan would rather spend their limited resources, be it the budget or screen time, on superfluous stuff like that worthless Missandei/Greyworm romance, or on the Sand Snakes, who are beyond ridiculous on the show - instead of concentrating on adapting the stuff that is actually in the books and that a lot of people care about.