There is a mixture of challenge-->investment-->reward problem going on here.
We are too broken to be faced with any meaningful threats right now. You never feel in danger, and thus you never feel like you are overcoming a hurdle or conquering something. Hence, the experience is hollow.
On the enemy side, there is no complexity and everything is randomized. You have health bars, health bars that turn off your abilities, and health bars that one shot. These health bars also approach you in randomized and haphazard ways instead of crafted, learnable engagements, so there is no need to ever learn an encounter or optimize it, where a lot of game knowledge and finesse come in.
Despite this, you can pressure players with sheer numbers of enemies, which is when horde shooters are most fun. Unfortunately enemy density in normal missions is just not there.
Challenge tends to drive investment. Umbral forma, arcanes etc are less desirable when you don't need them in any content. What is the point of maximising and perfecting builds if you never need to use them? If we did have avenues that posed significant challenges we would need to invest in to stand a chance these resources would become much more valuable.
What can you even reward right now that would appeal to players and incentivize endgame?
If we had genuine challenge high tier upgrade mats would be a suitable reward that would be evergreen. You would need to add strong nuanced gear that can be seen as tangible upgrades to existing gear used to enter endgame. You would need to add cosmetics. Furthermore, you would need to add a sense of prestige. Above all, you need to create the feeling of fulfillment and satisfaction.
These are pretty deeply interlinked. I don't see us being able to have endgame that is meaningful without core restructures in the game design tbh. For starters, you would need to massively curb player power by limiting unlimited acccess to stuff like energy for example, and we all know how that will be received.