I'm not following, if you use a centralised exchange *which is what we are discussing here) Monero would be taxed too.
Why would anybody be dumb enough to use a centralized exchange? We've had this technology for 15 years and the technology has not failed, but the exchanges around the technology have failed more than once and will continue doing so.
Because decentralised exchanges are sadly not ready for large amounts of money. I'm not defending centralisation, I hope we move towards that way. My point here is that bitcoin can also be used in decentralised exchanges, you don't need monero.
Of course, Bitcoin can be used on a decentralized exchange. However, most Bitcoin is obtained through centralized exchanges with no-year customer, and therefore reduces the anonymity of people who use decentralized exchanges, making Monero more of a need.