Well, that sucks.
As far as TV show endings go, you'd be hard pressed to find more universally disliked than how 'Game of Thrones' ended.
That said, you have to acknowledge the fact that the show was impossibly built up and that trying to bring a show and story like this to an end was always going to be difficult. Moreover, how do you end something like this in a way that isn't expected?
Well, in this case, you cancel out the two main characters and then put a tertiary character in as the king of the realm because... well, why not. Whether people predicted (no pun intended) Bran as the eventual ruler or not, there's no doubting that it's not the ending anyone wanted.
However, George RR Martin has previously said that the show's ending is its ending, and the book's ending will be different somewhat as they're working in two different mediums to one another. That said, he was the one who came up with the idea in the first place.
At any rate, the commentary for the final season of 'Game of Thrones' confirmed a number of rumours about where characters ended up post-show. Arya, for example, left Westeros to become an explorer while Drogon didn't eat Daenerys but instead carried her to Volantis.
However, DB Weiss and David Benioff also confirmed that Bran will be crowned as King in the book's endings as well. Peter Dinklage noted that, out of all of them, "Bran is the only probably pure person in there."
Well, there you have it. No need to devote yourself to reading 5,000 pages of purple prose just to get to that ending if you don't want to.