While this is by no means factual, this is an opinion of how this will play out.
At some point in the future, we will break through and quantum computing will become a reality and all existing encryptions will be insufficient and will be able to be broken in an instant with a quantum computer. This will not be an overnight process. It will evolve over time once we find a reliable method to manufacture quantum computers. During this time, we will find new algorithms for hashing and encryption that are quantum resistant. There are apparently some of these already in existence today – of course we don’t know if they are quantum resistant yet, but the theory is that they are.
This new quantum resistant hashing and encryption methods will be added to Bitcoin Core and new wallets will appear. Everyone who runs a node will want to update to this solidifying a change in the Bitcoin protocol. This will mean that we will have a new type of wallet which will be named when implemented. For now, we will call it QWallet. Those who have Native SEGWIT wallets (starting with bc1) and any other type of wallet before will need to be migrated into this new QWallet wallet. This will require the movement of all Bitcoin into the new QWallet before quantum computing becomes a reality. This is a good step because it will show what wallets are dormant (for example Satoshi Nakamoto’s) or lost.
Once quantum computers become reality, we will have the power to crack any lost or dormant wallets and transfer funds into a new QWallet (effectively stealing any Bitcoin within those wallets). This will bring back any lost Bitcoin into spendable wallets and we will once again have all 21 Million Bitcoin become spendable again.