I have to say I don’t think the command course should be easy to pass.
The level of skill, discipline and maturity required to take a multimillion dollar aircraft and 200+ lives to some very interesting and challenging places means that the command course should be stretching, it shouldn’t be easy, it should be challenging.
It should leave the candidate feeling like they can cope with any realistic emergency the aircraft can throw at them in a timely and considered fashion while maintaining a high level of safety.
Not all 5-10,000 hour FOs are able to demonstrate this.
What I would add is that if you have previously held a command at a reputable airline then you have demonstrated the required aptitude to be a Captain, and sometimes its better for the airline to take a very experienced Captain and type rate them rather than upgrade an FO. It’s quite easy to take a B777 or A340 captain and train them to fly a A320 or B737. It’s not that easy to take an FO and train them to think like a Commander.