The trouble with that is that if the fees are negligible compared to the economic activity that is mediated (as crypto bulls claim), then you need to lock a massive amount of capital in the collective float of the currency.
For example, the M2 money stock velocity of the US Dollar is historically in the 1-3 range, so that each dollar either in electronic or hard currency form changes hands a few times in a year.
So for Ether to generate even something like a meager 3% yield to beat inflation you need to charge on the order of 1%-3% for each transaction, which is orders of magnitude over what is expected or competitive. It follows then that the vast majority of the holders of Ethereum, and the backers of the scheme themselves especially, do not expect to earn from the fees, rather they speculate on the token itself, they hold it because it will appreciate in the future. They are betting ether will replace, say, dollars in mediating transactions and hope to gain value by depreciating the dollar.
To be very precise, I'm not saying that Ether can't move much faster and accrue larger fees (as it in fact did recently), but that the preference of the vast majority of non-speculators is to hold on to currency and near money substitutes for periods much larger than it would allow holders of ether to earn trading fees commensurate to the risk of holding ether. So the flurry of recent activity (and the willingness to pay large fees) must be of a speculative nature, unsustainable on the long run.