Validity of “Oxygen Running Out” Theory

After what I’ve read and learned during the last several months, I’m fascinated by the prevalence of the theory following the disappearance of the sub that it may have been intact for several days under the water with oxygen levels diminishing and the vessel and crew still salvageable.

Did anyone with even the faintest knowledge of submersibles actually believe this, or was it just a kind of vague, widespread, layperson wishful thinking theory?

I acknowledge that I’m coming from the perspective of a layperson who already knows what happened, but even so it’s hard for me to understand how anyone thought the Titan was just coasting along the ocean floor waiting to be rescued.