Google this / do your own research - this is not "FUD", this is just sober fact. Bitcoin public keys can fall with QC's of just 1556 qubits. (Source:
https://security.stackexchange.com/questions/33069/why-is-ecc-more-vulnerable-than-rsa-in-a-post-quantum-world ). Misinformation you hear is that it takes many qubits to crack RSA hence bitcoin is safe - this only relates to the bitcoin mining algorithm not the ECDSA algorithm used to relate public/private keys which is more vulnerable. Again - in 2 years or so IBM will have QC strong enough to reverse engineer private key from unhashed public key. When this happens panic will spread and bitcoin will crash. This is as predictable as the housing bubble collapse of 2008 and just like then, there are people who will shout "FUD" at anyone showing the plain and simple facts. Don't be on the wrong side of this.
Do not disregard that the developers are well aware of this scenario and I believe they will not get idle and just wait for these so called quantum computer to break the Bitcoin security. Because before that I believe there are lots of low security that will be cracked first giving a hint that there is a need of an upgrade of security for Bitcoin hence the possibility of your speculation or prediction might not happen.
I think as the quantum computer progress and develop, securities also evolve and develop. So it is a race and not just a one sided race where QC are the only one progressing.