The Fat Man told the producers how the books, and by extension, the character arcs, end. Also, there's a lot of things that were pruned/changed in the early seasons, like zombie Catelyn and Jeyne Westerling. Arya is still connected to/learning to warg Nymeria, Jon kinda sorta wargs into Ghost. Mance Rayder is stilt alive (although glamored by Mel).
I don't necessarily mind editing when its a series this long with a huge cast, it was poorly done in the last three seasons. The Pern series is even larger and not only spans a medieval setting with dragons and dolphins it has an advanced space faring element as well. I am not sure if something like it could be adapted to work within the confines of HBO (or any other network) without getting Rome'd.
Also, the first seasons worked because no one knew
if GOT would be successful and HBO would continue funding it. It probably would have gotten the Rome treatment as well had it not had the luck to air right as social media was taking off and there was an avalanche of Tumblr/Facebook/Twitter posts littering the internet. Even the Hillary campaign referenced it (remember
this?).
Besides hookers & blow is something they budget for, it's called production costs.
