This is my thought as well.
However, the thing is, I'm not sure how that reality will come into being. It's most likely that everything will go through a process before that happens.
Let's say it's 2030 and the one global digital currency (hopefully based on blockchain or something similar, but even better) has already happened. Let's say it's called U (stands for Unit). So now we value everything based on U instead of $USD. So now what? Don't we still value our cars, houses, properties, services, websites, domains etc.? I presume that we still do. We just globally value them based on U, that's all.
One global digital currency doesn't have to change our perception of value, rather just the medium with which we represent different values. Hence, I'd also say if at that time people still possess some form of crypto currencies, and they still use those for different transactions or at least value them for some other reasons, then things like Bitcoin, Ethereum, and other alt-coins will still have some value. With a few exceptions of course:
1. They's be valued based on U.
2. The fluctuations of their value (which is based on U) would be close to non-existent.
3. They'd all (most likely) slowly convert into U (like many other paper currencies) - this is what I called the process.
Of course, the probable worst case scenario is that there will be a one world disastrous economic collapse and one global digital currency will rise from the ashes. But other than a WW3 or another pandemic of apocalyptic proportion, I don't see what can cause that. I hope nothing like that will ever happen, but if that does happen, will there still be a digital world left that looks anything like it does today? - I doubt it.