Grind =/= Endgame...
Actually it's the opposite... You have grind in the whole game. And still you can say it lacks end-game.
Thing is, why should people do X. Someone said "oh fun"... Well rewards are fun. That's the whole cycle of a game, Challenge -> Reward. Warframe is somewhat lacking in both.
For the most part, the game is somewhat too easy. When it isn't it's just ridiculously hard because scaling is not very good. You can't say it's fun when the game jumps from you one-shooting everything, to requiring a clip or two of the best weapons in the game to down an "elite" mob, of which there will be around 10 in the screen at some point.
So factor one, make it more challenging, but fair. Don't just ramp the numbers to crazy and say "well, it's hard now".
And when it's challenging, be sure to reward players for doing well. Because, "bragging rights" only require you to do it once.
Arbitrations illustrate this issue perfectly, in my opinion. DE went to almost ridiculous extremes to make them feel difficult, they added invulnerability, they start you out near the "enemies will now one-shoot you" level, and they added constraints with the "rules" making the missions "harder". And finally there's the "death penalty".
Did all this make Arbitrations challenging? NO... In defenses it's down right boring, and feels more like you're just waiting for the next wave, and most other modes feel similarly boring, and you still need to wait for higher tiers. What it became was more frustrating, not challenging, or even hard. Only frustrating, because now you had to deal with weird interactions between invulnerable sentries and near invulnerable Infested Healers. Grinneer still have their hitscan weapons and one mistake and you're dead without even knowing what killed you (or you simply get a bugged out death, been known to happen). Then there's the frustration with the wait times, and the overall paltry rewards.
This is a issue that permeates Warframe. I love the game, mind you, i do, and DE is for me one of the best developers out there, and has their hearts in the right place. And honestly, CDPR, and maybe Rockstar, are the only other developers that are even close in terms of how they treat us. But this doesn't mean that they're infallible.
The rewards and end-game has been an issue with the game since it's inception.
I won't lie, i'm a late adopter. I liked Warframe, when i saw the first videos, but fell victim of the P2W rumours, so didn't even try it at the start (sure had i made the research, i'd probably play it for ages, or i might leave due to how it was kind of weird at the start - or so i'm told, but that's besides the point). I started playing after a old time guild mate said he was playing this instead of going raiding in GW2. So me and my brother came to try it out as well... This was somewhere in 2016, the main patch was Titania's Silver Grove patch at the time.
We loved the game, and so me and my brother got to work on the game (soon we surpassed both the guy that got us in the game, and another friend we later found out also played WF sporadically since launch), and by the time Nidus came around, we had already unlocked and mastered every Warframe and prime that had been released up until then, excluding some of the Vaulted Primes (at the time, Rhino, Nyx, Mag and Loki) and a chunk of the weapons. Sorties were kind of repetitive, and we didn't feel like we had anything more to do but wait for the next frame... So we took a break. When Octavia released we did the quest, unlocked the mandacord, and kind of drifted away from the game even before completing the frame.
I came back for a while trying to farm Rhino Prime and Mag Prime when they got unvaulted, but failed to get my friends to join. We only finally got back during this Tennocon, and now we had a lot of stuff to catch up to, which we kind of did to varying degrees...
I, personally, unlocked all other Warframes in the mean time, but Loki Prime, and i'm almost done with the weapons as well. Have mastered a couple of Zaws and most of the pre-fortuna amps, etc. And i'm already seeing my friends drift off a bit, even though they're not even nearly as close to completing everything as i am.
All this to say, that without a proper structured end-game, with it's own rewards and motivation for players to engage in, Warframe, even with it's quick content delivery will eventually fall back to the same issue. And there's a lot of people like me.
PvP, if they could sort out the servers and some different rewards might be the solution. Bringing back Clan v Clan could be another way to give end-game a meaning. Other ways would be to give players larger tasks to obtain gear. Better Leaderboards (maybe rewarding endurance teams with something more than bragging rights), like i said, maybe create milestones in missions, like survive 2h, get a Reactor. Defend 100 waves, get a catalyst, etc.
Even the AABC should be reworked. Using it in all mission types, immediately makes some missions more rewarding. That's why people do excavations when they can, it's way faster to get to C than a defence. So why is it that the rotation and the rewards remains the same, even though the effort and the investment are the same (except when all the power core carriers all get stuck and you have to waste 10m finding them).
I'd probably make different rotations on different game modes, and i'd distribute the loot tables so that you get different stuff in different maps. This would immediately make farming more consistent for the players, and would make the whole star map useful, instead of everyone congregating in the same few maps. Why, because if you know that on Io, for example, instead of Endo and all the 9 Meso relics on rotation A, you only got Endo and 3 relics, and then you had 3 other relics on rotation A in Europa, and 3 other on wherever. Then you'd pick the planet with the relic you need, and do that spot. With a chance of 25% per relic instead of 10% players would be able to find more consistency in their loot.
Same can be said for mods, why does rotation B need to have 8, mostly uncommon, mods? Why not put a couple on A? Or maybe focus mod rewards on some missions, and more relics on others.
Give players some agency to chose the content based on their objectives, making more choices valid.
At this point my thought process is: "I need Neo S9, so Valefor it is..." Because it's the excavation with the highest chance of Neo rewards (less mod clutter) on the star map. Now if missions were a bit more spread out, with rotations appropriate to their wave cycles, then i might go do something like a survival, which is more fun than excavations, for me, or maybe even spy missions.
I could go on, but this is already a long post, and it's late...