Centralised vs Decentralised systems
What makes one system harder to upgrade to post-quantum security than others? This section will explain the differences and implications of centralised and decentralised systems in context of the quantum threat.
First, In centralised systems, the developers have the control and authority to decide what to do with the system - for example, banks and websites. These can be upgraded to post-quantum secure applications if necessary, in a much faster way than decentralised systems like blockchain. This is because in every upgrade a decentralised system like blockchain, there has to be an agreed consensus so all nodes execute the same upgrade.
Second, speaking of blockchain - all users have to manually move their funds from old addresses to new post-quantum secure addresses. Bank users do not have to do this.
Third, lost addresses (addresses where the user forgot or lost the private key) will never be able to move their funds in case of blockchain. These will always stay vulnerable to quantum attacks. Banks do not have this problem.
These are 3 major downsides of blockchain in the context of upgrading to post-quantum security and have to do with the nature of blockchain itself - decentralisation.