cash cow of TV, they will milk it for years to come.
HBO needs this series alive in 2019, 2020, 2021 and beyond to keep their subscriptions up. So I imagine it's a case of HBO's stakeholders demanding GoT continues to run, rather than a legitimate premise for a prequel.
Can they deliver on a show with 40+ new characters(half of whom will die), and a plot which isn't just a rehash of ASOIAF in another era. There's a danger of it going the same route as 'The Walking Dead', one of the highest rated TV shows that sank into repetition and lost its audience.
What's funny about GoT is, the people I once would have told "Read this series/Watch this show... it's about knights, dragons and zombies..." They would have laughed. And those are some of the show's most committed fans now. Other adaptations of sci-fi/fantasy novels would draw in large audiences too if given a chance. It's where Netflix is winning right now, they're investing in every direction. Altered Carbon was amazing and now they're working on The Witcher (There's also a Castlevania mini-series already out).
Now Youtube is producing shows too, whoever hasn't seen Cobra Kai needs to get on it!
The race is on right now for licences to adapt every fictional series from the past century, it's where HBO should be investing their 5M/episode budget, and they can return to GoT in 2024 or so.