I do not think it that outrageous although far more likely is 3M users with 100 SP.
As more people join the platform, we are going to see the amount of the reward pool distributed over more accounts..meaning the payouts will be less. It will get harder over time to earn SP.
As that happens, I imagine communities will become more and more important. You can't possibly try to keep up with everyone, but if you are part of an interest-based community, it becomes a little more feasible not only to build support, but to be part of something active where people connect across a shared interest.
Either way, it'll be interesting to see how it unfolds.
=^..^=