a heart surgeon didn't start out in that role, a partner in a law firm didn't start out as a partner same for CEOs etc
Correct. But when that doctor started, was he assessed and employed as a future heart surgeon? - doubtful.
When we were assessed by the recruiters at an airline, were we assessed and employed as future captains. If they didn't think you were suitable for command, then you wouldn't have been offered the job.
There are a few extra command requirements such as total hours, or hours in company aircraft, but if every FO who's been in the company more than 2 years has the ability/personality/experience/whatever to get on a command course, then who do you offer it to? The simple answer is seniority.
Not only does seniority negate the brown nose effect, but in our job we need to be open and share information and experience with each other, and learn form others' mistakes as well as our own. If there's no seniority protecting promotion, then people will stop sharing the good information, and safety is degraded.