Command seems to be about being in the right place at the right time and very little else.
In 18 years and several airlines, only my current one seems to give any thought to preparing FO's for the left seat and that is done by making the operating sector the responsibility of the operating pilot subject to veto from the skipper if you do anything daft. And that works both ways too. This means that a command course can concentrate on command rather than things that you should have been doing for years in the normal course of things.
Previous airlines have either recruited direct into the LHS (cheaper), promoted only those with prior LHS experience (less training risk) or preferred ex military heroes regardless ( Training staff predominately ex military).
Despite a good system of mentoring, still dependent on right place right time which is a bit of a bugger when time is running out and all the captains are younger.
The only real answer is to get into an expanding company with a retirement bulge on the horizon.