Well, it's just like you can choose to not do the main quest in any of the games, but if you read the time lines and storylines on The Imperial Library, they say "unknown hero" defeated Jagar Tharn, went to Mantellan Crux to find Mantella to activate Totem to control Numidium, defeated Dagoth Ur, closed Oblivion gates and helped Martin Septim defeat Mehrunes Dagon. But you didn't have to do any of that stuff in game. So after TESV, we will probably be reading "unknown hero, who was the last DragonBorn, defeated Alduin and saved Tamriel from the World-Eater, restoring peace and prosperity to the land."