I genuinely want to know, what is "End Game Content"?

I see much discourse surrounding "no accommodation for end game players" or "no end game incentives" and im genuinely curious what these people are seeking. In my mind, its like asking League of Legends devs to add End Game content, like.... what end game?

We get a steady stream of new toys, new MOs, occasional new subfactions, is that not the whole heart of the game right there?