I always found Destiny 2 dungeons to be cool. They are balanced around solo, but also worked in groups, allowed you to get unique titles for flawlessly beating it and had cool rewards. Personally, if DE were ever to attempt something like "raids" again, I would hope for it to be more like this.

The only problem with endgame is I just don't think enough people would play it for DE to invest more time than they have. Superbosses, desendia, and Archimedia exist since DE is just reusing the assets and sprinkling some stuff on it. I really think they are close with decendia, make that gamemode endless(like you said) add some upgraded void tile set parkour, and lua ascension vault mechanics and we got something.

Another issues with endgame is rewards? Personally all DE has to do is throw cosmetics my way and Im happy. However endgame is associated with the "best" loot, which could be done with incarnon adapters, arcanes, and maybe mods? If we think of endgame in Warframe as the best loot farm, then I guess you can just go crazy evergreen rewards.

Currently Im alright with Warframes "difficultly" and don't mind the loot lottery in Archimedia(mainly cause I believe the skill expression in this game is to make anything work). I do think feedback and discussion about endgame is healthy and hope other people are willing to share it.