can any currency ever really have an enforceable commitment?
Not sure. Bitcoin used clever design and cryptography to solve many things we didn't think possible before, so full commitment could be possible.
why can't commitment and trustworthiness simply be tossed into the mix when currencies compete?
I think that's one of the points of the paper. Can issuers simply compete on reputation?
One thing that economists like to talk about is "commitment technology". i.e. Is there some way for issuers to verifiably prove that they can commit to their promises. The paper's result is that without such a commitment technology, competition is not enough to keep the market efficient.