If the showrunners had needed to stretch the story more, either Dany might still be over in Essos or Game of Thrones would be out of Kingdoms by the end!
Besides, more than half of the Seven Kingdoms are already in shambles, and the army of ice zombies approaches from the North.
At least the eight season total is a bit of a compromise between what George R.R. Martin (and likely HBO, what with Game of Thrones as their highest-profile and most popular series) wanted and what David Benioff and Dan Weiss felt comfortable doing.
As much as fans would undoubtedly keep tuning in for however much longer the series could have run beyond eight seasons, it may be for the best that Benioff and Weiss don’t have to stretch the story out further than they’d like.