I tried to come up with a coherent argument and failed. The best I can come up with is that with a growing DAC there consistently is USD sell pressure and BTS buy pressure in outside exchanges. The other way around (selling BTS to buy USD) is smaller than buying BTS with USD because people can instead park their money as BitUSD instead. This asymmetry, which must exist as long as the amount of BitUSD (and other BitAssets) are growing, should continue to cause an average net increase in the price of BTS with respect to the underlying real world assets.
I think what you are saying is that as long as BitAssets are growing, this will be positive for the perception of growth in BTS. And I agree with that. But there is no direct economic linkage apart from the fees.
No, I am not just talking about the perception. I am saying if the BTS/USD market was symmetric and there was a lot of liquidity, people could convert between BTS and USD easily without changing the price (same with BitAssets and USD or other assets because of arbitrage opportunities). For the price of BTS to be driven up, we need more demand for buying BTS with USD to get into the BitShares system than selling BTS for USD to get back into the fiat system. If people are just moving into the system to make some profits off speculation or gambling, or move money around using the internet, but then are ultimately cashing out into fiat to pay for real world goods/services, I expect the market to be mostly symmetric and therefore expect little to no growth. But if people instead keep their savings in the BitShares system, if they pay for goods/services in BitCurrencies and the merchants then pay their employees in the earned BitCurrencies, the BTS sell demand in outside exchanges will be significantly reduced (limited to paying taxes and other goods/services that haven't yet adopted the new technology) because it will instead go into the BitAsset shorts/longs (or even just holding BTS as savings). This causes an asymmetry in the outside exchanges which causes the larger BTS buy demand to drive the price of BTS up relative to USD.
Edit: Notice that nothing in my explanation says that the BitAsset growth causes an increase in the value of BTS necessarily. People could in theory just keep all of their wealth in BTS. So I guess I am not actually arguing against your original statement in the OP. But having BitCurrencies is useful in keeping the wealth in the BitShares system rather than being pulled back out into the fiat system (thus leading to the asymmetry that drives BTS value up) because BitCurrencies have price stability and BTS does not. This is one of the big advantages BTS has over BTC in growing the value of the token since people do not have to sell BTS for USD to pay for goods/services priced in USD (of course assuming the merchant can pay their employees and supply chain in BitUSD).