The purpose of xmr is to protect one's privacy. This is at least my understanding. I dont view monero as a bitcoin killer or even a litecoin killer. Its there with one main purpose, to ensure everone a right to privacy. Not everyone cares about privacy. So bitcoin is not going anywhere anytime soon. But there are people that value their privacy. For them monero is a perfect solution.
Also, there is no reason to use monero for tax evasion, or to think about it as being desing for this purpose. In monero you have view keys, to disclose the transactions when needed. So a merchant or a person can keep their transactions privet from other merchants or people, but be fully transparent to tax office to which he/she can provide its view keys.
Companies and governments determine what money they accept, and what company or government is going to want a money that links them to all their activities for all time?
Yes, companies and governments decide, and they will not allow anonymous or even psedoanynomouse cruptocurrency to be "officially" used which is not controlled by government/company.
In Russia, for example, they already want to
launch their own cyrpto, approved by government. The "BitRuble will not be pseudo-anonymous like bitcoin due to the aforementioned Anti-Money Laundering (AML) and Know-Your-Customer (KYC) policies pursued by the regulator".
Its super easy for government to force ppl to use their 'own' cyryptocurrency. Its enough that they will pass laws requiring merchants to accept NSAcoin or whatever coin (or employers to pay salaries with such a coin) that the government can track and control.
Not every country will do this. Some countries that care about privacy (like Switzerland or something) might go the opposite way and embrace something like Monero in an attempt to attract privacy focused businesses (like shapeshift) who are seeking a new home.
In the future people may immigrate to new countries for privacy reasons.