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.
If we break Bitcoin by eventually convincing half the world to use it as their primary currency and payment system, then we fucking won.
I'll go ahead and agree with that as well. But I don't see any reason not to try to stretch the life of Bitcoin-I out as long as practicable, however, especially if it just takes a certain amount of planning and is considered in determining development priorities.
When Bitcoin-I's race is run, I imagine that what will form is a deliberately feather-weight foundation upon which as many 'bitcoin-1 like' shards as are needed can exist. Each 'shard' would contribute some small portion of it's resources toward securing this foundation solution. In doing so all would help support one another.
Trading value between the 'shards' when needed should be fairly straightforward and reasonably easy on the users.
In this way I would hope that a scalable peer-to-peer based economy can exist and require nothing more than the most primitive of network resources thus making it that much more difficult to attack.
I outlined some ideas along these lines a while ago before losing interest. If anyone is interested, they are here: http://bakcoin.org