I'm pleased that it's worked well for you but I'm sure we've both seen more than one Ponzi scheme and more than one bubble; they always look sustainable too, right up to the point where suddenly they don't. Even the cryptocurrencies nominally backed by physical assets are unproven (in the sense that nobody's yet collected a pocket full of gold after the linked cryptocurrency failed) which makes it perfectly possible for their value to drop to zero, and we don't have a long enough track record to be able to assign any reliable probabilities to their future behaviour.
Your statement above sounds worrying like "this time it's different" - and I'm sure we've both heard that before too!
Quite the contrary; this time it's exactly the same. Every correction in the cryptocurrency market is followed by a recovery, one which eventually surpasses record highs, before the cycle repeats. Take a look at the price charts over the last few years and you'll see that pattern repeating over and over again. Iron-out the fluctuations over the longer period and you'll see the price is only heading in one direction.
Don't confuse cryptocurrencies with the Ponzi schemes that use them. While it may be easier to setup and operate a cryptocurrency-based Ponzi scheme, cryptocurrencies are no more to blame for their existence than fiat currencies are for the the Ponzi schemes that utilise those.
Your concerns about the 'unbacked' nature of cryptocurrencies and the risk that they could somehow 'drop to zero' are very common and typical. Everyone who uses cryptocurrencies (myself included) comes up against those same psychological barriers initially. How can money be created from nothing and how can it possibly have value if it isn't backed by anything? When you've spent most of your life, as we all have, believing that all 'real' money is controlled and backed by governments and banks, it's very difficult to see beyond that programming. We all think we know what money is. Only when you start to question and study the origins (and future) of money do you realise that it's not what you thought it was. And to someone who is yet to push through that barrier, that probably all sounds like nonsense -- I know, I've been there -- but believe me, once you start to understand how and why cryptocurrencies came about, the underlying mechanisms and the reason they have value, it's a real eye-opener.
And when you do begin to understand cryptocurrencies you'll start to compare and question the value of fiat currencies, which are 'backed' by nothing more than governmental promises (ones which are continuously being broken though the gradual devaluation they call inflation). When you notice your money devaluing and realise how little those promises mean, there's a realisation that ultimately it's fiat currencies that are on the road to zero, especially now that we have cryptocurrencies to replace them. This has already begun to happen in countries like Venezuela, where the people have chosen cryptocurrencies, backed by consensus and hard mathematics, over fiat currencies (that have collapsed in value due to corruption and broken promises).
What you eventually realise is that the value is in adoption and usage. And that applies just as much to cryptocurrencies as it does to fiat. Just like Facebook has value because millions of people use it, the value of a monetary system is linked to usage. If enough people use it as a store of wealth and agree it has value, the value holds. The value will never drop to zero while there are people using it because someone will always be prepared to buy when it hits what they believe to be a bargain price. Likewise, as adoption grows, and the finite number of coins need to represent a larger market capitalisation, the coin value will rise. And that's precisely the reason we see an increase in value over the longer term. Unlike price spikes, which are largely due to speculation, adoption drives the price up at a gradual and sustainable rate.
