It seems like all the worry about quantum computers breaking encryption is based on speculation that someday quantum computers will break modern encryption. I think we can all agree it will happen at some point, but I have yet to find any credible information that gives an accurate timeline for this. "Experts" say 30 years, but where do they come up with these time estimates? Do we even know how many qubits we would need to break RSA or similar algorithms? I haven't been able to find any credible information to answer these questions.
Let's say we will have a quantum computer in 30 years that can break RSA, what type of data is even worth grabbing now that will be relevant in 30 years? Are there any instances of a "harvest now, decrypt later" attack yet?
Thank you for your input.