3e had Greyhawk as the default setting. 4e had "points of light".
In the 2e and 3e days, "the Great Wheel" was the default cosmology with several products referencing it.
Should 5e have a default setting or cosmology?
I personally don't want a default setting or cosmology. I think the rules should avoid mentioning anything other than the bare basics of cosmology. (e.g. The spell "Contact Other Plane" would imply the existence of other planes, but doesn't say what the planes would be.) That would encourage DMs to design any setting or cosmology they want without having feel like they have to shoehorn them into the "official" D&D setting.
What do you think?
Yes.
1. For new DMs and players.
2. For the shared experiences across the hobby.
3. For experienced DMs and players that don't have time to make it all up.
This does not impede people from using another setting, not in the least. But it does enable entry to the game for new people, and w/o new people, there is no DND eventually. It does give people who don't want to invent a new world, a world to work with. It does give us a shared experience.
For all the talk of the importance of story, I keep reading threads that ask for no story in the game. And I don't get that.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.