It’s worth nothing that withdrawing custodial support from NBT/PPC and NBT/BTC will force volume to move to NBT/USD.
Say there is only custodial support for NBT/USD
If you want to purchase NBT, then you will get hit with a bid/ask spread if you purchase NBT directly using BTC or PPC. You can minimize this slippage by first converting BTC to USD and then converting USD to NBT. This will lead volume to shift from NBT/PPC,NBT/BTC to NBT/USD.
tl;dr; You cannot wait for volume in the NBT/USD market to develop before withdrawing custodial support. If you don’t withdraw support from NBT/PPC and NBT/BTC, then the NBT/USD market will never develop (or at least not for a very long time).