If half the world were using bitcoin everyday we would be living in a completely different world so those numbers are meaningless. That world would likely have vastly superior technology. If bitcoin breaks because its too popular/useful thats a good thing. Bitcoin2 will fix the issues and we will be better off in the long run.
In my last post, I agreed with evoorhees's 'we fucking one' philosophy in general, but I do take some exceptions with certain of your suggestions as quoted.
Firstly, I think it likely that a shift to Bitcoin would coincide with a loss of confidence in other currencies (and conversely, that Bitcoin will continue as a fringe spec of dust on the shop floor of the global economy absent such a thing.) Such a loss of confidence could happen fairly rapidly and fairly soon (though hopefully it won't.) What it is unlikely to do is to wait for technology to develop to the point of supporting a much more resource intensive Bitcoin network.
Secondly it is clear that technology marches forward, but the latest advances are typically available to select entities for a long period of time (and often indefinitely. Think about satellite. Sure, we enjoy PPV porn on demand, but we do so at the pleasure of large corporate and government agencies. Ping me when a terrorist network (which the Bitcoin community is one legal decision away from) launches it's own communications satellite and uses effectively for more than a few hours.