By the final season, it had four remaining writers: Dave Hill, Bryan Cogman, and showrunners David Benioff and D.B. Weiss. I’ve done casual research on all four (meaning, I wikipedia’d them, googled them, and read/watched a few interviews and such), which I’ll summarize here:
David Benioff: Benioff is most experienced writer of the four. He has an academic background in writing and literature, and holds an MFA in creative writing. He also has screenwriting experience, having written the screenplays for (among others) Troy, The Kite Runner, and X-Men Origins: Wolverine.
D.B. Weiss: Weiss has a published novel, called Lucky Wander Boy. He has also apparently worked on multiple screenplays (one was based on Ender’s Game, one was a proposal for an aborted HALO movie, and one was a prequel to I Am Legend), but none were ever produced.
Dave Hill: He worked as an assistant to Benioff and Weiss early on in the show, then was promoted to writer in the fifth season. I couldn’t find anything else he’s worked on.
Bryan Cogman: Studied acting at Julliard. Appears to have had no prior writing experience before his hiring.
So, of season eight’s four writers, only one of them had ever actually had a screenplay produced prior to Game of Thrones. Two of the four appear to have had no professional writing experience whatsoever at the time they were hired. Does that seem right to you? It doesn’t to me.
The conclusion I draw from the above is this: good TV writing requires that the show take writing seriously, as though it’s a craft honed through experience and care. This conclusion is treated as obvious in other areas of production. They didn’t, for example, pull some random guy off the street and ask him to play Tywin Lannister, or design the show’s costumes, or manage the lighting and sets. And yet three of season eight’s four writers had, as near as I can tell, exactly zero screenwriting credits between them prior to Game of Thrones.