I'm not sure why this has been ignored, but there is already a very challenging firefight/survival mode in Halo 1. Near the beginning of Keyes, if you stay at the opening of the hole before dropping in, Flood will start to spawn and drop down from the hole in the ceiling in the hallway.

You have no cover, and there is nowhere to run but down the hole or across the hole with a grenade jump, at which point they stop spawning. Why haven't people posted videos of themselves trying to survive as long as possible before dropping into the hole?

When the spawning starts, you would be hard pressed to survive even one minute.

well, 1st off ppl didnt know firefight mode til H3ODST showed up.
2nd halo gamers thought it's only an AI bug.
3rd, some didnt even cared/noticed.

however, i stayed through that area until i ran out of ammo

Funnily enough, the actual idea for Firefight came from Halo 1. Not from the part you're mentioning, but from Truth and Reconciliation once you get into the ship and you have some waves of enemies attacking you.