I believe the issue with Warframe having no "endgame" is that DE are doing what they know, which is making a console game.
This game pretty much consists of all of the element's you'd have in your standard console game; Rough beginning, make your way through acquire and upgrade, then kill the boss. It's just done on a larger scale in Warframe. Judging from that point of view, what else could they do but scale enemy armor/damage and add endless missions? aside from adding more "kill the boss".
I remember in an old dev stream, they were trying to classify the game based on a question and c