Halo has been struggling for decades. Not dying, but certainly struggling. And one of the clear reasons for the franchise's fall off since Halo 3 is that Xbox has been losing the console war. There just aren't enough players out there on Xbox for Halo to meaningfully compete with non-exclusives like Call of Duty.

Slipspace, the game engine that 343 Industries designed to power what was supposed to be a new generation of Halo games, going beyond 2021's Halo Infinite, has been dropped by Halo Studios (formerly 343 Industries) for Epic Games' Unreal Engine because Slipspace would have made it harder for the studio to port Halo games to PlayStation and other platforms,

It's recently been revealed that the Halo series will be moving to Unreal Engine 5, which may support a long-running franchise rumor.