BItUSD interest isn't tied to growth of BTSX. That's where the misunderstanding was.
A BitAsset can be defined in terms of any clear consensus point. By default we have BitUSD which is consensus tied to the dollar. We can also define it to be any easily calculated (unambiguous) equation relative to a well defined value. IE: a weighted basket of commodities, or an interest bearing bond. Think "Bit30YearTreasuries"
I have two assumptions:
(1) There will be a demand for different BitUSDs (BitUSD-5, BitUSD0, BitUSD5 (= BitUSD 5 %), BitUSD10, BitUSD100 etc.).
(2) shorting BitUSD is going long BTSX and going long BitUSD (buying BitUSD) is going short BTSX.
Therefore...
If BTS X is believed to grow much for sure then many will want to go short BitUSD and the demand for going long BitUSD will be small. Therefore the BitUSD shorters have to offer the BitUSD long positions much interest so there is enough demand for BitUSD long positions to meet the BitUSD short positions.
If BTSX is doubted to grow much or if the growth of BTSX is doubted at all many will want to go long BitUSD and woun't be able to get much interest....
Conclusion:
Depending on the growth potential of BTS X the demand for BitUSD long and short positions will vary. Therefore, depending on the growth potential of BTSX, sometimes BitUSD0 and sometimes BitUSD50 will be what traders will agree on to trade.