But TV production just exists in this paradoxical world where it's hard to imagine it ever working like that, for a couple of reasons:

1) Most of the time successful shows are either in the mode of open-ended storytelling that could stretch on indefinitely (because everyone is happy making the show and getting paid) or they are in some kind of breakneck sprint to wrap everything up in about 10-12 episodes (because the network, showrunners, and/or actors are ready to move on).

2) Even when there's a clear agreement about the timeframe for ending the show with theoretically more than enough lead time to brings things to a satisfying close, it's still pretty complicated! After season 3, ABC and the Lost showrunners made a plan to do 3 full seasons to bring that show to an end after 6 years and, well, it wasn't exactly smooth sailing to the finish. They fell into the trap of continuing to expand the mythology of the show in seasons 4 and 5 (because that's more fun and interesting than beginning to gradually contract the story) so they were still left trying to hash together an ending in season 6 that met their objectives for how to end the story while trying to "answer all the questions."

So even if Weiss and Benioff had agreed after S6 to do a full S7-9, I'm not convinced they would've been able to execute it as methodically (and nicely) as the roadmap you laid out above. With 30 episodes left to play with, it would've been pretty tempting to spend the first 15-20 episodes doing more cool stuff, introducing new characters, etc., and just trying to figure out how to land the plane in the final season.

This Is Us on NBC is going to be the next big show to deal with this exact scenario. They've done 3 seasons and the creator just signed a deal to produce exactly 3 more to complete the story. I suspect that, given the way the show has kind of borrowed and modified the Lost format of storytelling, they will have similar issues.