What if there were some totally unique service or thing that you could ONLY buy with cryptocurrencies conveniently? Something that could not be duplicated. That would be enough to remove the ponzi "smell" from cryptocurrencies.
Gold and silver have unique molecular makeups. There are some things that gold or silver can do that other things simply can't do. That is separate from the fact that there's a limited amount of these metals in the Earth's crust and separate from the fact that they are shiny. Therefore, gold and silver can't be considered ponzis. As long as the materials themselves serve some useful purpose, they will hold value.
In theory, we don't need anything besides speculation for cryptocurrencies to hold value. They can hold value if we all just "decide" that they hold value. But this is a collective mind game that may not stand the test of overwhelming universal bearishness or mass panic (which inevitably and periodically arise).
We have to keep searching. It is not enough to "just get a GUI" or identify a large potential market.
Fundamentally, we really need a clear reason why XMR will continue to exist and hold value for the foreseeable future. Are we waiting for the entire global government fiat system to collapse, create a vacuum, and for cryptocurrencies to then fill the void? That seems very unlikely (as there will always be at least one government left standing that people trust) and it's something we have no control over. What's more likely is that a killer app propels cryptocurrency forward and people voluntarily switch over from inferior alternatives. What's more likely still is that cryptocurrencies sorta just languish for a while like battery technology. (The first true battery was made in the year 1800, but the first practical use of batteries (to power telegraphs and whatnot) didn't come around until after 1835).
https://en.wikipedia.org/wiki/History_of_the_battery#Invention_of_the_batteryI will say that
if the global political environment becomes such that
major-league tax evasion is necessary for everyone (rich and poor) just to survive, and physical cash and gold coins are banned, and every visible form of economic activity becomes strangled by rentseekers, then cryptocurrency will become very attractive. But frankly, that world doesn't sound very fun whether your cryptocurrency or secret gold stash has become worth a lot or not.
/musing
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.