It's not exactly "different sized people mess it up". It's not just different lung capacities, but different lung compliance-- which also changes over the course of disease.
Most simple pressure regulators will not fare well in the environment, either, because of the bidirectional flow, etc.
The "Y" hack definitely has use, but finding matches and keeping patients matched definitely means we will only be able to use it sparingly.
The flow direction issue is partly addressed by the unidirectional valves in many ventilators/anaesthesia machines. The bigger issue is staffing expertise, I fear.
Most simple pressure regulators will not fare well in the environment, either, because of the bidirectional flow, etc.
The "Y" hack definitely has use, but finding matches and keeping patients matched definitely means we will only be able to use it sparingly.