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.


Oh and now a quick response to the original question:
I don't think we'll be seeing destructible tribe stuffs for awhile. Right now the combat engine is ... uhhh... yeah. Along with the fact you cannot make a secure perimeter around your land. It wouldn't make sense to put it in now.
Best way to implement the changes are defense first, then offense. Or 2nd best implement them both at the same time. But if they implemented all offensive parts of siege before players have the ability to fortify their homes... then you'll just have one big landmass of crater=)