100%How much of the casual gamer audience should they attempt to reach out to?

This does not mean that they should make an inferior or less fun game. If this is what you are afraid of I would say that the game could absolutely be even better and more fun than what there is now.
I would say they have been trying to reach as many a pocket of the hardcore gamer crowd. Optimized board-minis game, digitally enhancable through a subscription service and still running on the most familiar old fashioned style of rulebooks and setting books.From the design of 4E, whether explicitly or implicitly, WotC may have possibly been trying to reach out to the MMORPG crowd. Though how successful they have been so far, is unknown.
I believe you. This is why the game should be build for short or let me say casual attention spans. Rules that are build to make creativity come out as naturally as possible in the tabletop environment.If I was a kid today and went straight to MMORPGs like WoW, I don't know if I would even be attracted to pen and paper rpgs. Pen-and-paper rpgs may seem quite quaint in comparison to WoW, especially for kids with very short (or zero) attention spans.