Marathon has had a rough start, launching to reasonably positive reviews but suffering from dwindling player counts mere months after launch. Bungie released the seasonal Vault Breaker mode that shifted the game's structure to be more PvE-focused, and subsequently revealed a permanent PvE mode would arrive later in 2026 after Vault Breaker showed promise.

Bungie has now confirmed the new PvE update, named Symbiosis, has been delayed to December 8, after being promised for autumn 2026. Symbiosis will introduce Marathon's first permanent PvE experience, providing a new way to explore, progress, and engage with the world beyond traditional PvPvE extraction gameplay.

Symbiosis will also include a refreshed Perimeter experience, a new social space with a firing range, a new Runner shell, an experimental PvP Team Deathmatch mode, and improvements to the new-player experience. Bungie describes the PvE mode as "only the beginning," using player feedback to evolve Marathon into a "more alien and dynamic survival experience." Following Marathon's launch, Bungie also suffered "significant" layoffs at the hands of Sony, suggesting the project may not be as financially successful as needed.