I do not know about other cryptocurrencies, but the designer(s) of Bitcoin were paranoid that one of their encryption algorithms might be broken, so Bitcoin uses 3 different encryption algorithms on top of each other: a public private key algorithm and 2 cryptographic hashes.Regarding the security of the network, what about quantum computers? Can’t they, if created, or if already exist secretly, hack these cryptos?
Quantum computers can crack the public private key algorithm, but the execution time to decipher the 2 cryptographic hashes remains exponential (and thus practically unsolvable) for quantum computers.
If the public private key algorithm is broken (when quantum computers are powerfull enough), your Bitcoin wallet is safe for as long as you have not transferred money from it, since the public key of the wallet is only exposed when you make a payment from your wallet. If the algorithm is broken, someone can calculate the private key of the wallet from the public key and anyone who knows the private key of a wallet can make payments from it. If you have only received money on a wallet, the public key has never been exposed, only the hash of the hash of the public key. If you are afraid that the public private key algorithm will be broken, then every time you transfer money from a wallet, you have to transfer everything on the wallet: the money you want to pay to someone else's wallet and the rest of the money to another one of your wallets. Every time you make a payment, you have to create a new wallet and transfer all your remaining money to it.
If you follow this practice, Bitcoin can only be cracked if we have quantum computers and if the 2 cryptographic hash algorithms (SHA256 and RIPEMD160) can be cracked. The designer(s) of Bitcoin were afraid one of the 2 hashes could have a backdoor. (SHA256 was designed by the NSA and published as a standard by NIST. Much earlier, the NSA modified the DES algorithm before publishing it as a standard and it appeared later that the changes allowed the NSA to crack DES. Sometimes you should not trust encryption standards issued by the NSA.)
You are safe only if at least one of the 3 encryption algorithms cannot be cracked, but you can never know for sure that is the case.