It is a very common misconception to view the marginal XRP costs of a transaction as important (right now transactions cost 10 XRP). The real cost is the cost of opening a new Ripple account, which is 200 XRP. This is orders of magnitude greater.
Right now it is, but that's because people aren't using it. If Ripple was to take off and people actually used it for something useful, the transaction cost is what really would eat up XRPs. After all, one person only needs one Ripple account in their life*, but they'll presumably conduct thousands of transactions in their lifetime.
*: Add any that would be used by businesses.
Transactions would be expensive if the supply of XRP were scarce, but it wouldn't be scarce just because the founders kept half of the supply.
You are of course correct about this as long as nobody uses Ripple. If transaction costs and reserves start eating up XRPs, those who hold XRP have to sell it at a cost that keeps the Ripple system competitive with its competing systems. If they don't do that correctly, their hoard becomes worth next to 0 as Ripple fails.