I suspect that inside this miracle of technology there will not be a system of cooling the substance to zero, but technologies to conquer magnetic fields for the same purposes, which, as the scientific press writes, are developing.
I sort of agree with this. The reason we need QCs to be cooled almost to absolute zero is to reduce decoherence. Cooling is a (partial) solution to a problem. There may be other solutions where cooling is not required (or where a smaller amount of cooling is sufficient).
Everybody looks the wrong way when they think about security issues.
There are billions of accounts on the darknet that are sold for nothing. We are all hacked a long time ago, and so we will in the future if we keep the old key encryption technologies and password (and biometric) authentication methods.
I sort of agree here, too. A system is only as strong as its weakest link. That weakest link often turns out to be human errors or laziness. However we can't really argue that a) people will make errors or be negligent, therefore b) there is no point to implementing secure cryptographic systems.
Why do you need a quantum computer to attack a bitcoin - I don't understand at all. Even the old asymmetric cryptography on elliptical curves, with a 4-fold increase in the length of the key - will remain a dream to crack the known algorithms on quantum computers.
Because the power of a QC scales exponentially due to superposition and entanglement. Superposition meaning that a qubit can be - to simplify somewhat - both 0 and 1 at the same time. Entanglement meaning that multiple qubits can be combined into a single state. So the number of classical outcomes that can be assessed scales 2^n. The nature of QCs means that they are strong on integer factorisation and the discrete logarithm problem (both normal and ECC). Shor's algorithm can dismantle
current asymmetric cryptography.
There are as you know various quantum-resistant approaches to asymmetric cryptography that offer potential defences against a QC, however these do also bring new challenges such as increased key size.
Going beyond bitcoin and cryptocurrencies, one common assumption is that there is no danger until a sufficiently powerful QC appears. This is not the case. Quantum-safe security needs to be implemented as soon as reasonably possible. I am quite sure that people are storing today's encrypted traffic for the future, so that it can be decoded once a QC is available. Anything communicated by public-key today can be deciphered tomorrow.