Depth in open water: when to have a redundant air supply

Hey guys, What is your thought on depth when it come to redundant air supplies or a stage Tank in an open water dive? What Depth (air) wouldn’t you go if you wouldn’t have redundancy?

Background: I’m not about to do it in the near future but am thinking about doing a recreational deep-dive course as a start for going tech. (As it’s mandatory for advanced nitrox etc. as far as I know.) But the thought of going down 40 meters with only one air source scares me a bit. (Not taking doubles into account) Talking about an shtf under water and somehow one managed to go out of air and (accidentally) got into deco sharing would easily get you bend as an emergency ascend is required. Or isn’t it? Not having enough gas for the depth doesn’t necessarily scare me but not having enough air to safely surface (maybe including a possible deco stop) does. (Meaning I’m not overly scared about running out of air if a second tank or stage is available.)

What is your thought on the topic? (Asking in r/techdiving as you guys should know how to evaluate the risk better)