The issue is that there's not enough to do at each park to fill a whole day. That means, there really isn't enough to entertain a family for a full week.
The Magic Kingdom has almost twice as much to do as any other single park in Walt Disney World. Filling out the rest of the parks in an equal manner would not necessitate more vacation days from people, but adding more parks would. As it stands today, Magic Kingdom is a full day park, Epcot is perhaps somewhere between 1/2 and 3/4 day, and both Animal Kingdom and Disney's Hollywood Studios can be seen together in one day. Three days for four parks. This is part of why Magic Kingdom is so packed - people gravitate back once they're done with the other parks.
Avatar is part of the mission to solve that problem at DAK, as is Rivers of Light and the extended hours for Kilimanjaro.
The changes announced today will of course renew interest in DHS massively, but that doesn't mean there will automatically be enough to stay busy at the park for a full day. We will have to see. I think the people who are concerned that so much has closed and only four attractions have been announced to replace them have a potentially valid concern.
I agree that trying to grow beyond a one-week destination is not a good plan, but right now Disney needs to grow its Florida parks back into a full-week destination. Today has demonstrated that they are clearly on the right track, but they have not yet demonstrated that the problems will be solved. That will come with more details - timeline, attraction menu, entertainment and retail . . . It's great that Star Wars Land is happening, and it looks beautiful, but how it is done means more than just what it will look like. How these additions are done will determine their worth.