To achieve command at 15 years would require a fleet change, and it must be abundantly obvious why shorthaul would not have appealed.
In my time, the longhaul fleets had their fair share of RHS 'sitters'; who preferred a senior position and accompanying life style with little esponsibility to having to work for a living.
BOAC
A few years ago I had a chat with a similar RHS sitter and he stated that he could not afford to go for promotion as he had 1st call on the "best" routes.