It is my belief that the Compendium will be this. An online SRD for the base rules may come eventually but I believe it will be later when they have worked out how to leverage the online fan community a-la Gleemax and the VTT.Sounds great in theory. Does 4E have a SRD available to DDI subscribers? If so then I haven't found it yet.
I did ignore rules that we didn't need. Once we started needing them where were they? In a rules splatbook thats where.
We are in a transition period at the moment, and not just in D&D
The essentials will be there as pointed out by DEFCON1 to recruit new players and the Compendium will be the basic rules references. Pretty much everything else is in the various builders.
One thing that will impact on it is how fast the world in general is going to transition to purely electronic publication.
I suspect that within 10 years or maybe even less very few people will be buying books in paper format. And those that still do will be buying hardbacks only for the pleasure of owning a physical book.