The main purpose of XRP is to prevent spam. So, when all XRP have been distributed and then destroyed as transaction fees, how is the network supposed to work???
XRP protects the network, acts as a vehicle currency, and funds the development and promotion of the protocol and network.
Not all XRP will be destroyed as fees. XRP is, likely, lost more rapidly than it is destroyed due to fees. The fees can be lowered. And like BTC, divisibility makes losses not a concern. XRP is currently divisible to 6 places.
Learn more about XRP here: