Is there a valid reason for not splitting this into 2 properties?

I mean, this has always been a pain, and it's great that they are refactoring it. But the way a block behaves and the way it treats its children really are two distinct concepts.

If I have a list of items, I may very well want the same "outer display" and a different "content display", and yet, I'll have to define outer display everywhere.

Not a massive issue, but display: block flow could be a shorthand for display-outer: block; display-content: flow. In the same way margins or padding work...