In the interest of moving forward in some way... something I find interesting in some of these conversations is the expectations some have of the DM.
They're supposed to create an entire world. Continents, nations, governments, and so on. Groups of people with goals and hierarchies and rivals. Individuals with a place in society and a connection to others. And history for all of this stuff... how a nation came to be, why a city was founded where it was, who begat who begat who begat who, and so on. Social elements, too... migrations and movements not just for different cultures, but for entirely different beings. Elves and dwarves and orcs and so on. Each with their own narrative needs.
Add to this a cosmology... different divine beings and possible other planes of existence. A way for all these things to fit together and make some kind of coherent sense. To have each serve some kind of purpose and to provide a structure to it all. Gods and devils and so on to give the cosmology some kind of agency in the setting... representatives of ideas or concepts in the physical world. Entire religions devoted to these ideas, many often at direct conflict with one another... each with their own practices and goals and rivalries and holidays and so on.
In most cases, a magic system that flows from the cosmology in some way (or perhaps alongside or even counter to it). Ways for all the different facets of magic to work, and how they influence this world. Who can use magic and how? What can be accomplished with it? Different types of magic or different schools. Organizations related to the study of magic, or maybe the suppression of magic. And so on.
And, all of this stuff is meant to be compelling from a game perspective. A lot of it isn't relegated to mere color... if there is a prison plane such as Carceri, it serves a purpose not just for the setting, but for play as well. Perhaps the characters need to break into the prison plane. Perhaps they need to escape. Perhaps they send beings there, and those beings hold a grudge. Each of this myriad of things is not just there for flavor, but there for play.
All this is expected of the DM.
But if they let the players introduce anything... it all comes crashing down?