That argument has been levelled and Bitcoin, and I personally agree with it. It explains the adoption boom up to this point, and off the back of it it's possible to fairly confidently predict a bust when returns fail to materialize. Right now returns are based on adoption coupled with increasingly constrained supply.
In theory, when bitcoin reaches a saturation point of adoption, the value of bitcoin won't increase anymore (it can't if there is no more demand). At this stage deflation would technically have stopped and the currency is matured.
This should lead to a much more stable economy than we have now, where people aren't encouraged through the use of cheap credit to make emotional purchases for things they don't need, sure the economy might not "grow" at the same rate but the growth would be a more precise representation of what people truly needed (instead of just being a big party). Maybe people would even lead more stable responsible lives.
That's one of the reasons currency decreases in value, it's a deliberate effort to get people to use just about anything else as a store of value.
> Won't people rationally flock to such currencies
Supply constraints combined with utility to drive demand (whether a supply constrained “currency” has the latter is another question) makes for a good store of value—investment—but that's orthogonal to utility as a currency.