So the more MicroCash you have the higher the percentage of the fees you get. It doesn't necessarily reward early adopters, it rewards people who have MicroCash.
MicroCash: Taking from the poor and giving to the rich.If I understand correctly, if an address has a greater-than-average amount of currency then it will get interest.
Paid for by all the accounts with a less-than-average amount of currency.
So there's an incentive for everybody to (try to) be above the average. That seems like a recipe for a lot of disappointed people, especially since a graph of size-of-account will have a long tail with of lots of little accounts.
Making a half-thought-out-prediction, I'd say the end result would likely be one big centralized account controlled by the largest exchange with over 50% of the currency in it, because there would be a natural incentive to store your money in the biggest account so you get the most interest...