My thoughts on this, and the longevity of the show.
If we look at other productions of books to tv/movies. You had LOTR, Harry Potter and Game of Thrones. Those are the biggest I can think of. All of them large productions that took a long time from start to end of the series, and they all followed the origin stories rather faithfully (at least in the beginning, GOT stands out in the later seasons since there was little to no material to draw from). The original fan base created by the books helped create hype for the franchises and their continuing instalments. The original viewer count is massively skewed towards fans of the books.
So loop goes
1) As people who haven't read the books get into the movies/show, the hype is kept up by the adaptation being "fairly" accurate to the original content. This keeps the fans of the series involved and going "oh, you think it is good now, just you wait" to new viewers. The fans of the books help with the hype as they enjoy watching the books come to life on the screen. This marketing is worth more than any promo or ad.
2) This led to a re-release of the books and which more and more read, increasing hype for the franchise of both movie/series/book format. You all remember how in 2001 you'd have to go deep into a book store to find LOTR under the fantasy section, but after the release of LOTR, for the next three years they were front and centre of any bookstore (along with Harry Potter).
3) As new fans of the show started reading the books that were now marketed as much as the show, the books were "better", but still the shows were faithful, and the people who didn't want to wait for the "next instalment" suddenly became the same same as the people of point 1. Fans of the book content.
Following that all three of those franchises exploded in popularity. Films/series created fuel for the books, books created fuel for the film/series.
I don't see how this series can follow that loop.
The massive view count of the series is created by the brand name. I am also fairly certain the original viewer count is massively skewed towards fans of the books, who have read the source material. But that is where it stops. Fans of the original books can't say "Oh, just you wait!" because they no longer can tell. It's deviated from things from characters, character motivations, physical characteristics and entire plot events. If they manage to do that within a few episodes, where will the series be in one, two, four+ seasons from now? The red wedding of GOT was well known and people had set up cameras to prepare for the events to tape their friends and loved ones cries. GRR Martin said he loved that.
Falme, the blowing of the horn, we come, the ogier fighting at the stone? Who knows what "favourite" moment is cut out. What character growth / motivation is removed. Mat is my favourite character in the books, no idea if he would be in the TV show. Or if he will face events or do things that made him my favourite character to begin with.
Way I see this going is that the viewer count of the series will continue to drop. Soon I expect the viewcount will skew far more towards non-book readers as book readers. And unlike the previous examples, as they pick up the books to not wait for the next season they will notice just how bad the show is compared to them, and also stop watching. The shows deviation from the books will create a negative loop of viewers unlike the positive one of LOTR, GOT and Harry Potter.
Not sure if the creators of this show are aware or just don't care.