It's a good question because I think the answer for BTC is to rely on the fees, which POB doesn't have. POB is really just layer 2 though, the DPoS consensus happens on Hive, so the PoB mining would not be necessary to sustain the POB token. However, if we wanted to keep proofofbrain.io's distribution going maybe decimal precision could be increased or @proofofbrainio could fund it.