> Every staker increases their stake by the same percentage; each one is just holding on to their share of the pie; there is no exponentially growth.
Aren't you missing here that there's a certain threshold below which holders practically do not stake? All those people with low balances do not get this "same percentage".
Maybe you can compare it to a savings bank account. It is mostly small amounts of money which reside in pockets or wallets or piggy banks. They don't earn interest as the larger amounts in savings accounts do. Most people with little money do not get any interest on the little they have, whereas wealthy people often manage their wealth so that they have gains.
So I claim that there's indeed an exponential effect that will shift the percentages over time. And it should lead to accumulation instead of distribution.