FUD.
You don't think the devs of a $150b asset would let it go to "0" do you? If anything like that starts to possess a threat to Bitcoin, there will be either a HF, or a quick fix but it will be fixed.
Of course! Bitcoin is a code that can be updated. All you need is consensus among miners and you can fork BTC and shape it to whatever you desire.
Quantum technology progresses so slow that they are going to have years to think of something. Also, quantum computers will have difficulty adapting to the currently used software. They don't see simple calculations like our home PCs and need to be able to have the data prepared for them to be able to process it.
Modern PCs have problems with opening old DOS applications from the 90s. It's the same with quantum computers and our standard software just the gap is much bigger.