I think their design philosophy is also probably playing a part in dropping player counts. The progression treadmill is severely hindered by their wish that weapons be weak to encourage players to depend on stratagems. Their passes are filled with lackluster guns and armor that isn't really all that different from what players already have. They add a few new stratagems here and there, but not nearly enough to keep people grinding for new ones.

I would argue that a sizeable number of players can only grind out the same missions over and over with little tangible progression to show for it for so long.

That plus their ever growing pile of technical debt.