I agree it would cost too much to research all teams from all nations and I don't think there will ever be an FM version where all leagues would be available. This is why: - adding the possibility to add leagues with the editor - adding the possibility to manage clubs from other nations, even with grey players could offer some workaround for those who can't find their favourite league in the game.
What about someone in Morocco who bought FM but can't manage his own local team? The Hungarian league was not always present in CM/FM (don't remember the exact year when it was added), and I was not happy about it when I was not able to manage Hungarian teams. Even though I probably would never manage teams from all nations, but still sometimes I like to load all nations and have a career game where I spend 1-2 years then I go on to another team in another country.