You don't take into account the social factors. People having a ton of tokens can easily freeze majority of them in the stake. People having a small amount of tokens won't be able to do so - first because they need them for a living expenses, second because they will be hit by transfer fees much more that people moving big amounts. This is all fine, but the system is deflatory, so this discrepancy will only increase and rate of it will also increase over time (accelerate). Basically big holders at the moment of the PoS switch will be the only big owners forever in the human history, and no new big holders can appear ever again.