Literally everyone and their mothers dog bought the war bundle for war and the stalker armor, come to think of it I was the only one who didn't buy it out of the 6 of us including me that played heavily back then, releasing interesting cosmetics with a huge new update was a smart move on their part, it can be done but piggy backing off tennogen is not a good idea when they have creative talent there to be tapped into.
Your claim is false though, it can be done and they have shown they can do it.
Also releasing no content for months on end is not a good idea considering warframe really has no replayable content besides maybe arcanes and that other game mode that requires a saryn or nuke equinox. Getting healthy game updates> Getting cosmetics shoved down our throats for months on end. Unless your into cosmetics which is good for you but I Guestimate a very large portion of the playerbase is here for the actual gameplay.
To address your second statement it is very true. DE has gotten lazier and lazier over the years opting to just shove cosmetics down our throats more often because........ well it sells and they get to be lazy about it which creates stagnation of updates etc. and the "why should I try hard when the money keeps coming?" I don't really see way in which this could be remedied easy, it would basically take us to stop buying cosmetics or for them to start dropping massive amounts of players to light a fire under them and get them moving again but that will probably never happen but i guess someone could dream maybe? Anyways yeah it is a big issue.