I don't know if the analogy would be appropriate but I thought of this question. Let's say that a vehicle was considered a "mission critical" component of an operation such as a freight shipping company. Without it functioning as it should, the mission could not be met. Would it be considered acceptable for the hood (bonnet, 
) to be locked with a key held only by the car/truck dealer?
How then would it be acceptable for closed-source software to be used in a "mission critical" system if the user could not "get under the hood to change the oil?" There may have been a time when automotive vehicles could only be serviced by the manufacturer, but that time is long past. Aftermarket or custom parts are considered appropriate and desirable even in "mission critical" systems. The widely held mindset about proprietary software must now be broadened and become more customer-centric. Open Source allows for this freedom of choice.