A 51% or higher computational power would be too easy for them to conduct an attack on the network.
Well, I'm not a technical guy and I really have no clue how the core developers can adjust on this threat.
Quantum computer poses a threat only on the encryption. I misread the article below..
For some references since the OP didn't include any:
Quantum Computing Becoming Real
Quantum Computers Pose Imminent Threat to Bitcoin Security
On a side note, so it would be possible also to generate a complete character case sensitive bitcoin address using a quantum computer.