By definition, any and all features that would add a parameter to said "giant-initializer"s would be difficult to add without further increasing the complexity of the existing APIs.
To your point about forcing the community to adapt: that's just not true. Any package marking its manifest with an older version should keep working, just like they do now. Until you need the new feature you don't need to change anything, as we saw with SPM's resources feature, to name one.