So, combine Question 1 with today's "Legends & Lore" (
Dungeons & Dragons Roleplaying Game Official Home Page - Article (Head of the Class)) and it all fits very nicely.
Basic and Advanced.
Except, they leave the door open for Advanced 1 (AEDU) and Advanced 2 (Psionics style powers) and whatever other types of advanced (Return of Vacian magic?).
Perhaps one problem is that in 2008 (ish) the initial system had to deliver an experience that was already advanced. Would the internet have exploded if they delivered Essentials in the original PHB? I think the "WotC is dumbing down D&D" catchphrase would have been more persistent and it might not have captured as much of the 3e audience.
Fundamentally, as they always stated, they faced the problem of getting new players through the learning curve while still being appealing to the old players. There is also the problem of people who want simple experiences and complex experiences. 2e addressed the issue (perhaps by accident) by starting with Basic and then went to Advanced and then eventually Players Options. I think 3e's philosophy was be everything to everyone... except that if you want a basic experience you have to place classes A, B, and C. Advanced 3e options came from how 3e was meant to reward "system mastery", where picking classes X, Y, and Z OR multi-classing got you a more custom and complex experience. And, of course, 3e didn't get it exactly right, which is why we have 3.5.
My view: I get the design philosophy stated in Answer 1. It is an elegant solution to a complex problem and will potentially solve it well. However, it doesn't
appear that this philosophy was in place from the start. It
appears that the original PHB provided a base system with self-consistent math and consistent power levels for any character option. This pays dividends as they can hopefully build whatever experience players desire on the new rules chassis. They invested these dividends in the on-ramp/basic experience (Essentials) and hopefully will further invest them in exciting new toys. But was that ever the initial intent of 4e?