The ending will be bittersweet, not tragic. That is what GRRM has said from the beginning, as the ending of LOTR was bittersweet. That means there will be sadness and death, but it does not mean that every single character will get a brutal, depressing ending. Frodo gets PTSD, never fully recovers from his experiences and journeys to the afterlife with the elves...but Sam gets to go home to Shire and start a family with Rosie. That's life. There is good and bad. That's also true of A Song of Ice and Fire. There is good and bad that happens and that's how the show will end. Many will die, but some will carry on and serve as the bearers of a new future ("a dream of spring") in Westeros. The ending is also coming from George R.R. Martin's own outline, so Benioff & Weiss are not crafting their own ending at all. As always, they are adapting GRRM.