It has been repeated often that the gateways can set the transaction fees. This is true up to a certain point. At 10 "drops" per transaction, the most they can lower the fees is by 90%. Further reduction in fees would then only be possible with a change in the protocol to add more decimal places to XRP units. If Ripple takes off, those who own the bulk of the XRPs can simply hoard them to drive up the price. This would put pressure on gateways to lower the transaction fees due to artificial scarcity of XRP. But the most they could lower the price is by a factor of 10.
There is grave concern about the centralized control over the XRPs. This concern is legitimate.
I have to admit that this is a concern that never occurred to me. But it seems implausible in the extreme. There are a hundred billion XRP, or a hundred quadrillion drops. Let's say 1/10th of a penny per transaction is small enough that nobody need be concerned about it being a limiting factor. For one drop to be worth 1/10th of a penny, the total value of all XRP would have to be one trillion dollars.
If that ever did happen though, the divisibility of XRP could be changed by consensus. Alternatively, we could use a scheme where you prepay for a number of transactions and then can perform transactions without a fee until the account's "transaction credits" drop to zero. The number of transaction credits you got for one drop could be adjusted by consensus.