This principle guarantees that at the very least one will not be criminally persecuted for acquiring, holding or selling crypto assets if there was no law criminalising these actions at the time when they were executed.
This is the sum total of the brilliant idea that you have made because it is also like saying that you don't put something on nothing and expect it to stand .
About the inscription of digital currency in the constitution, I'm sure that countries who want to pursue the taxing of cryptocurrency would start amending the areas that deal with financial laws in their constitution.