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
Crypto currencies provide a very important solution. This is the ability to send money at a distance without the need for a trusted third party. When the buyer and seller can meet
to complete a transaction one can use fiat cash but this is not an option when for example the buyer and seller are in different continents. As for the market it is those who cannot get or use a bank account, debit / credit card, Paypal etc for the transaction for what ever reason. In many cases the reason is poverty on the part of the buyer or seller or both.
Edit: There are many innovative money transfer systems that work only within one country. It is when international boundaries are crossed that fees and intermediaries make relatively small transactions say below 100 US very expensive.