One would be promoted the same way as in any other industry, 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, however if any of them does not like the t&c's at their current employer they can leave and don't have to start back as an intern or resident. My doctor was horrified when I explained seniority lists to him.
As for me if I could join another airline at current rank or even one step below back at home I would in a heart beat. for me to start back at the bottom would mean a 66% pay drop and our company knows it. Yet if they knew that there cpts or senior FOs could leave and get jobs elsewhere at similar ranks I am certain the negotiations would be much more beneficial to us. The catch as I see it is we need to get rid of seniority industry wide, even if the unions wanted it I would bet the airlines don't want it.