Forgetting Google's mistake the last week, I have a speculation concerning the future. Think about it.
Google indexes literally billions of pages. This is a daunting task.
What if, hypothetically, Google decided that any page on any site that doesn't have an IBL, should be considered supplemental. Note! Not a home page without an IBL, just lesser pages within that site.
This philosophy would allow Google to not index so many pages every week. Thus, Google doesn't work as hard. In their mind, everything is good.
Therefore, more site's pages are supplemental. Why? Any subpage without an IBL isn't that important. What do you all think?