I'd have to disagree with your #2 from the designers standpoint. One of the major design goals of 4E was to make it more of a class-based system than 3E was. It was a design decision not to cater to the people who prefer a more class-free method of character creation.
From the replies thus far, I may not have been clear. What I am advocating would not be radically different, in play experience, from the 4E we have:
1. Classes, class abilities--all that stuff--keep it firmly in the traditional D&D camp of levels. Or make it even more firm than it is now.
2.
Inside the powers design, acknowledge more fully that they are built on a mathematical model and reskinned, and take this to its obvious conclusion: The list of powers could be built from a component subsystem.
Couple that with an expanded ritual system, and you don't even need multiclassing. You either reskin your power to do what you want, or you step (partially) outside the class/power framework to get the skills, feats, or rituals that you need for the concept.
I say "partially", because with a stronger focus on the class part, it would be fine to make certain skills, feats, expanded rituals, and yes, powers, more promising for certain classes.
I understand Delta's argument, but note that he is making a different assertion than mine, rather than directly countering my point. He is saying, "D&D players want class-based design". Which I largely concede, but not to the same degree. What I am saying is, "When you do classes, do classes fully. When you get away from class-based design, do that fully as well."
I can't recall, off-hand, a game system that has pursued such a hybrid strategy: "In this limited box, you are free to mix and match components all you want."
Another way to summarize, in regards to the OP, is that the conceptual "silos" in 4E are clear cut and useful, but they didn't push those silos to their obvious conclusions.