I watched GOT from the start because I was already a fan. Tons of people I know had never heard of it until at least S2, 3 or even 4.
This, I know loads of people who watched it, but it was mainly book fans that did so at the start since they likely had already heard lots about it being worked on.
The first season was actually pretty slow for the first half it wouldn't have hooked many non fans at that point, but come the middle and the end it was great.
It nothing unique, I mean unless your glued to tv schedules etc its hardly unusual that you will fall in love with a TV series later than the start.
I actually watched the whole of Stargate franchise after it was first aired.
I know that it grew and grew, thats why I said it gained massive viewership. Over 2.5m per episode is good numbers for a cable network show in its first season let alone a fantasy one.
Really? Hell even things like expanse on Syfy managed 0.5M
NCIS was like 12 Million or something
The witcher S2 had something like 2 billion minutes watched in the opening week, with WOT also scoring highly in minutes watched (they seem to have moved from the viewing figures they used to quote to this kind of metric now, probably more accurate)
But really the point was that the book fans would have been heavily inclined to watch from the start.
The non book fans picked up over time, and as non book fans they would have had no idea if the Ned stark death, the red wedding etc was per the book or different.
Just like people who hadn't read LOTR 50 times wont know all the small snippets of information, or things like surfing elves were not a thing in the book.
Just like people who hadn't read WOT wouldn't know how significantly changed loads of things were.
These people really don't care about the colour of a hobbits skin, they don't care about characters being switched in events, they dont care about the addition or deletion of a character etc etc
They watch it on its merits and vote with their eyes.
These are the people the networks want to attract.
I think its great to see people attempting to create more fantasy on TV. Sure I would rather its close to the original source, but if its a choice of getting purple elves surfing on shields whilst multi firing their bow, or nothing at all, give me the purple elves