It was known from about 2015 that those 2 were losing their passion for the project......yet they still refused to quit and let someone else take over. No idea why they weren't just kicked out.
Saying that, (75x) hour long GoT episodes are never going to have the re-watchability of a 25 minute long sitcom (x200), to the general public.
Actually, you're 100% right.
When D&D decided after Season 6 that they were going to end the series within 13 episodes spread over the last two seasons, this series was fucked. I called it then, and I was 100% right.
If D&D, after Season 5, said "We're getting burned out, lets do this for two more seasons while bringing on two other producers on board for on-the-job training to take over for Seasons 7, 8, 9, and 10" it would have been the much smarter move not driven by ego.
GRRM and HBO said they were down for 10 seasons, and that's what the story needed to be completely fleshed out with all loose ends tied, minimum 9 seasons. End the series at an even 100 episodes.
"Oh, but HBO didn't want to spend the money for all the CGI and keep the cast for another 25+ episodes." THEN THEY SHOULD HAVE KILLED OFF SOME OF THE MAIN CHARACTERS, and nobody complained about the lack of CGI in the early seasons (in fact there was alot, but it was subtle rather than spending tens of millions on CGI dragons). If the dragons were expensive they should have adapted the 'less is more' philosophy rather than make each episode seem like a Marvel movie.
Even if HBO was scared about spending and extra $100 million on each season, they could have entered a partnership with Amazon to stream the series on Prime after a while. I'm just spitballing that idea but I'm sure there was a dozen different options available for investors or loans.
If the new producers for seasons 7-10 knocked it out of the park, yes, it would have been the 'Friends' or 'The Office' for decades to come, and fans would have bought the series box set on BluRay, 4K BluRay, and whatever physical media is developed in the future.
In other words, HBO fucked up and washed BILLIONS down the drain, because Game of Thrones could have been the new 'Star Wars' after Return of the Jedi as far as merchandise and license agreements.
Instead, its practically a dead franchise. Very few fans are interested in the new GoT spin-offs.