There is a paradox you need to know about blockchain based cryptocurrencies: the size of the blockchain itself is a big threat.
All transaction(s) are stored in a single file. It's the case for the majority of known Cryptocurrencies like BitCoin where the blockchain is about to reach 250 GB now :
Indeed each transaction done by everybody is stored in a single encrypted file where the next block has the checksum of the last block to validate the conformity of the blockchain, it's the blockchain model.
More people are using the blockchain - Ethereum has a contract for example which is a great piece of hopes - , more the blockchain is rising.
To understand why it's a problem, you need to know that Blockchain runs thanks to "nodes".
With 2 nodes active you have a peer to peer (p2p) based protocol enabled where the 2 nodes are synchronizing the blockchain itself.
It means to speak frankly that they are downloading all the blockchain in the server.
A node is more or less a server even if in IT 's terms stuff could be more complicated but we will simplify like that.
When your blockchain is growing, the need for empty space in each node is growing too. If you were running a private node connected to Ethereum for example, you should have now at least 150 GB of free space in your server.
In a personnal computer now it's not really a big problem finally where you have Terabyte easily but in a server, it's another problem.
The best server and costless one costs around 40 USD monthly to host a 200 GB hard drive with a good speed.
You can grab one with 100% SLA and very flexible plan at upcloud for example (clicking on this link we will offer you 25 USD in credit too by the way)
With such inflation in ETH (Ethereum), you started in november 2018 with a 10 USD monthly server with UpCloud and you are now at a 80 USD monthly server.
Then you need to multiply by 8 the money in 6 months you want to invest in the node and i don't speak about all problem linked to process correctly the node like the speed of your server. More blocks you are processing, more you need speed to process them.
Then it's becoming harder and harder to have amateur's node supporting the blockchain.
The first impact is the number of nodes may decrease with the time while the number of people adopting your crypto will raise.
Big cryptos are trying to setup foundations which support the network adding themselves nodes but there is a bias to this: this foundation are becoming more or less with the time like central banks and finally are centralizing what it needs to be decentralized.
The adoption of cryptocurrencies is, then, threatening the spirit of the cryptocurrencies itself.
The philosophy of cryptocurrencies will have difficulty to survive with a massive adoption due to basics technicals problems which are inherent to cryptocurrencies like we know them now.
Then, for example, if a big bank like UBS is opening tomorrow Bitcoin account as some people push them to do, they need to deploy quickly a lot of servers to help the network to sustain the demand then it's not clear that they have a real interest to do so.
We don't speak about extreme volatility and time for processing confirmation which will take back the banks dozens of years before today.
Then the first threat which is empeaching cryptocurrencies to reach massively the public is the blockchain itself.
The other danger is the exposure to spam and attack.
Indeed if you are injecting, legally, a lot of transactions to the network, you are attacking it.
If you are able to send massively transactions you will block quickly the whole cryptocurrency process.
Monero's developers in their scenario say that in 3 days, it's over: all the nodes of their network are wiped out then we can say that a large part of monero's coin is just lost.
The first result of such congestion would be to push the exchange rate to climb very high until liquidity will stop to be possible and then crash after.
The attacker needs than to have access quickly to its own liquidity schema, make a big injection, wipes as many as he can nodes and just wait for the final crash after he successfully exchanges all his own coin to fiat currencies (USD, EUR or Gold better).
We have a clear paradox which, for me, condemn cryptocurrencies of the first generation (BTC, ETH) to disappear or to become the money of the elite restricted to some fortunate client for luxury goods.
During this journey, the initial goal of cryptocurrencies which has been to be a popular exchange tool will reverse and will become the only privilege of some.