That's the point, you can't have so many balances in so many exchanges, because, in that case, each return is going to be very small. Just to take profit on an ETH USD arbitrage, you have to have 25% of your capital in ETH and 25% USD in exchange A, and 25% ETH and 25% USD in exchange B. So, when you take advantage of the opportunity to buy ETH at low price on exchange A and to sell on exchange B, you are only earning an arbitrage profit on a 25% of your capital.
And that profit become less and less if you divide your capital into more coins and more exchanges.
