Perhaps my biggest issue then is I'm not all in on the purely digital world that you describe and admit doesn't really exist, yet. That is to say, my plumber doesn't care about any of this and just wants cash. In the future, that can and probably will change, of course.
But today, in our current world, very few industries and virtually no blue collar industries accept such currency.
So then the question becomes, what is the value of <insert coin here>. Some will talk about energy, or efficiency. Some will talk about scarcity. Some will talk tech merits. But nobody to date has been able to convince me that it has any real value. There are no armies or economies validating it.
I think in simple terms, perhaps I'm a luddite. If someone, say completely disconnected from modern conveniences, were selling an item, I could perhaps trade physical goods for it. Or perhaps shiny metals, and explain why they're valuable(assuming they didn't know). Or explain dollars, and the guarantee behind them. How would you sell them on cryptocoins having value? The tech doesn't matter a ton here to a person, so onto the value. Why are bitcoins worth more than say, beanie babies of yore? Both seem to be run purely on speculation, at this point.
Said another way, if someone gives me 10k in cash, I have faith it will still be worth 10k in a year(ignoring our awful inflation). If someone gave me 10k in bitcoins, I have zero faith it would be worth anything tomorrow.