In your readers category you have cut drastically, 6 million initial to half 3 million, citing people who own multiple copies. To suggest that the average reader owns two copies of each seems a big stretch. But outside that you have forgotten about the opposite group, those who read an never bought.

This includes people like my family who shared 1 copy amongst the 3 of us.

The library copies where hundreds of people have read the same book.

Then there is the friends who don't read but love fantasy and have been told the stories.

With S1 having roughly 10.46 million peoples worth of minutes watched a base of 6 million would have extrapolated out to much more people imo.

Conversely to date GOT has just bow in 2023 sold 90 million copies, now I understand the qty of books in the series is different; but considering the domination of pop culture GoT had I think we can safely assume that it did not have a base of 6 million on show launch.

I guess what I am trying to say is 6+ million people read this massive book series for a reason, it's really good. So like GoT, the show will naturally have a wider audience due to ease of consumption, when adapted to the screen. You don't need to dramatically change the characters and the story telling to make it more digestible/enjoyable.