BitcoinMechanic recently posted on youtube discussing exactly this and echoes a lot of what @Scoresby talks about in this post and both argue why sovereign permissionless medium of exchange is what ultimately defines bitcoin's properties as hard money. It's an hour long, but the key points are made after 46:36.

He talks about gold and how even though gold isn't really used as a medium of exchange, it still has scarcity and store of value properties due to its molecular properties being enforced by the laws of physics.

But because bitcoin is digital, it's scarcity of 21 million and SoV properties is only maintained when people are actively using it and enforcing the rules embedded in code with each interaction/exchange. And if an overwhelming majority of bitcoin's economic activity is through third parties who then get to define the rules, then bitcoin's hard moneyness is vulnerable to capture.

Key Quote: "Bitcoin is not hard money unless it's used actively by people who are asking permission from no one. That's the only way it maintains it and I mean that with regards to storing it, taking custody of it themselves, and using it as a permissionless medium of exchange. If they don't do the latter... and the trend is away from using as a medium of exchange, then ultimately 21 million is not going to mean anything."