That kind of decision obviously couldn’t be a spur of the moment thing, considering how vast and complicated the production process was for the final battle episode and the rest.
As such, it makes sense that the seeds were already being sewn in the series’ earlier seasons, even if those seeds never grew out.
First, let’s unpack that George R.R. Martin was having conversations four years ago with showrunners David Benioff and Dan Weiss about Game of Thrones potentially getting to wrap up its epic, tragic narratives on the silver screen, as opposed to a truncated HBO season.