I think people confuse "endgame" with "the end of the game".
The term "endgame" actually has nothing to do with actually ending the game.
It refers more to your chars development, and level of success in the game.
When you start you have basically nothing, no idea of the world around you. limited stats,equipment, knowledge, etc.
When these things start getting to a very high level for you, then what you do after that point is endgame.
The ones who said "sandbox has no endgame, we'll be building more villages and/or making constant repairs and new armor/weapons/etc"
Guess what you just described.... endgame stuffs
Whatever aspects of the game there are to do to occupy your time once your char is maxxed (hard or softcap). That is endgame.