Just yesterday I had a conversation with a person who studied economy and finances at a University. He said that any Economic system to work propoerly needs some inflation. Since Bitcoin is a deflationary currency by design he said there's no way Bitcoin would replace all the money in the world.
You should ask him next time what he means by "working properly"
Easy to prove if your shoving Keynesian nonsense down people's throats, "working properly" could define the conversation, and if your friend thinks Keynesian theory, or the way governments currently run is "working properly" and well, then that would be the place to start the conversation, otherwise it'll be difficult getting anywhere with the other stuff.
Our systems of governance are pretty poor and and our politics hasn't advanced much from monkey/ape/primates in the wild tbh. The best crafters of political systems in history in my opinion were not overconfident and designed with human beings limitations and failings in mind.
As for digital vs physical - good money is fungible that is easilly divisible and transferable.
With the blockchain structure allowing for digital scarcity that is enforceable, digital takes the cake over physical.
I do sometimes worry though that some tight a55 proficient at being innept government will use try to use digital currencies to be more controlling, intrusive and hungry to monitor and pry into people's lives.
The people running this world are, by and large, a bunch of oxymoronic selfish idiots, massive and intrusive gov surveillance causes more fear and induces more anxiety than idiots who blow themselves up around others and do other insane and very difficult to stop acts. I'd rather live in a world with less control, monitoring, fear mongering, and inept foreign intervention/meddling/wars and a higher risk that some crazy might try to kill me for no good reason, because honestly it just seems like a very very low probability in comparison, and if someone is determined to do that, they probably can, not that they won't be taken in soon after and get away with it, fortunately they won't, additionally a tiny tsa and smaller focus and spending on antiterrorist stuff and our bloated and silly militaries would free up a lot of the budget to actually get on with the business of living well.