Income tax has become a dilema especially in cryptocurrencies however speculating about how government will take how much percentage of tax due to how much coins we have or trade doesn't sounds really fair since decentralized and annonymity were included and theres no way to deduce it someone really owns it or even trade it at the first time. I don't live in US but if government really apply cryptocurrency taxes in anywhere then they have to make sure they can calculate and measure how much someone really worth first.
It will be hard for the government to track how much does one Bitcoin users have. I guess it might be applied on some exchange but if that certain user are a holder, the government will just let him pay a small fee that will be unfair.
Some users especially the whales usually transfer thousand of Bitcoin annually, changing from wallet to wallet, if tax will be applied, it will be only for exchange and not for transferring BTC wallet to wallet.