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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.
Fungibility rather than privacy is the killer application; however one cannot have fungibility without privacy so they go hand in hand. I remain convinced that Bitcoin has a fatal flaw that has been exposed by the 1 MB blocksize limit debate, namely: How can a fee market develop in the absence of a block reward? Granted this flaw will not show up for a few decades but it one of the reasons why there is not a "simple" solution to the blocksize debate.