And again, what is this contra to?
Are you suggesting that players [for example] harvesting mine nodes in Rift at level 60 are participating in the "endgame" by the popular & vulgar usage of the term?
Would that be honest application of the term as you want to use it in this discussion? If so, then sure, players collecting herbs and participating in Keg Brawl are doing endgame activities just as well. If not, what would you set as the parameters of Rift's endgame as an example?
Otherwise, endgame is exactly as described; the final stages of a game [before completion] when limited pieces or options are in play. A term that is applied & practiced across many game forms consistently.
A game ends.
What you do in those final steps before then when only a few moves are left is classically
endgame.
That is all an endgame is.