Bitcoin's "value proposition" is complex. First, it is true what mindrust said here:
the code can be copied easily by anyone. (open source) There are hundreds of altcoins which can do exactly what bitcoin does. So %100 of the value of bitcoin is coming from its users.
So Bitcoin's 21 million unit limit has not to be understood as of an "absolute limit" for cryptocurrency, but as a limit of "shares" from the particularly "Bitcoin" variant. That actually is the strongest one and will probably continue to be.
What are these "shares" now? Well, here is where "usefulness" comes in: Bitcoin is useful because it has advantages over other money forms. Trustlessness, international potential, and (still!) relatively low fees. So Bitcoins are essentially shares of the network of users that use it because of this properties - merchants, remittance users, and last but not least, speculators and "savers".
But as of now, if we would base the actual Bitcoin price of the "actual use" as a currency, then Bitcoin would be overvalued because it's simply not used very widely as a currency but speculation is the major "tool" of it. So a part of Bitcoin's value is also an estimation of its possible
future use.
Only if Bitcoin conserves an advantage with respect to this future estimation over other "altcoins", then it can also work as a store of value or "digital gold". It isn't digital gold only because of the 21 million limit, but also because people believe it could be a major world currency in the future.