Op here, many responses here, so i'll share some more opinions.
I think that Warframe is mostly pretty good, there are questionable design choices, for sure, but i feel like the game reached a dead end, not in the sense of being on life support, but more as in ... it already offered what i expected of it.
I started playing around the release of ... Equinox ? If i recall correctly, and back then, you'd clean the main quests, jump in the void with your homies and pray for those sweet rare parts to drop, while extending your dojo as a side activity, quite nice and simple. I don't think any new planet, warframe or quest will live up to that level of expectation many people have with the game nowadays.
Second Dream was dope, but because it was new and unexpected, something surprising, and this is not something you get to pull off on command.
So, there's only so many damage numbers, packs of enemies and flashy guns that will keep me invested in a game i already enjoyed throughfully. I wish challenging activities were given a bit more attention, such as Pvp (i quite frankly enjoyed conclave for some time), raids and what not, but it did not happen, can't be helped.
With perspective, i really can't say that DE pulled their punches trying to expend the game and keep things fresh, but hey, it doesn't feel like playing space ninjas no more, and i don't think constantly focusing on the game's weak points (that i simply used to overlook because i was too occupied having fun), will bring the experience back for me ... and that's fine.