Quantum Computer can't stop the high security of Bitcoin. There will be no additional security on Bitcoin than the highly recommend securities of Bitcoin. You can secure it offline and online with non custodial wallets. And don't keep your long term investment Bitcoins in exchange or custodial wallets.
No. You are wrong. The threat of quantum computers is real and just so you know, it doesn't only pose a risk to BTC, but any cryptographic system; the good part is we are at least 20 years behind the reality of this threat.
Yes, based on the theory, quantum computers are a threat to crypto any cryptographic system as you say. But in reality and so far, there has been no real threat from QC and damage to the crypto industry. Everything is still theoretical so I agree with the view that QC cannot prevent or harm bitcoin in particular and the crypto industry in general.
As you said it will take QC at least 20 years to pose a real threat to bitcoin but we need to know that bitcoin will not sleep for the next 20 years either. It will also be upgraded and developed to keep up with the development of technology at that time. So clearly the threat of QC is still just theory, rumor, nothing more.