I think the real issue comes from advancing the timeline. Every single faction and even their subfactions had a last, desperate gambit to either save the galaxy or doom it in the worst way imaginable.
The Necrons were on the brink of awakening to purge the galaxy. Chaos was ready to burst forth from the Eye of Terror and claim reality. Across the stars, Orks were rallying under Ghazghkull’s great Waaagh!, and the Tyranids were only just beginning their endless hunger.
It was the midnight hour for the Imperium and the Eldar any one of these threats could have spelled the end of everything. It was a lot of fun to think about and talk about. But instead, a handful of Primarchs return with shiny new Space Marines, and suddenly every other faction is pushed aside, reduced to little more than punching bags to make the poster boys look good.
Honestly, they need to give every faction their own MacGuffin or goal, something worth chasing that could, if achieved, plunge the galaxy back into peril. Just sitting around waiting for “big things” that never actually happen is dumb as fuck.