For me, the way we could still make it possible is to establish a local exchange wherein, if we convert our money to crypto, these local exchange could already have the portion intended for the tax of each transactions.
This is the other way that they can tax bitcoin and other crypto transactions but there could be other ways that we haven't think of. It wouldn't be decentralized anymore if they will require everyone to be registered as a bitcoin and cryptocurrency enthusiast because it's against from what bitcoin is intended for. But we have no choice if this is going to be the system that they'll implement.
They'll ask for the record of that local exchange and file taxation for those users that have made profit on that exchange. If they'll not voluntarily go to the tax bureau, they'll be the one to inform the user that there's an associated bitcoin tax record that should be paid.