According to Zero Linden, the project was motivated by thoughts a year ago when Second Life was growing rapidly and Linden Lab was thinking about how to scale the grid.
There have been two timeframes set back then: The next few months and the next years. This architecture discussion is about the next years.
If we assume that virtual worlds will be a significant part of the future, then the following numbers might not be so far off:
* 16 million regions (probably more)
* 2 billion users
* 50 million concurrency (on whatever device)
These numbers will not be possible with the grid right now and additionally some more use cases need to be solved like connecting 3rd party hosted regions to the grid.