If I am right, blockchain is a DB design that follows the "Secure by Design" guideline.
But we know that in DB development, there is a wider range of possibilities that could be considered "secure".
What makes a blockchain-type of DB more secure than the others?
Will BTC always depend on a blockchain or there is a possibility to get it migrated into a better design?
Interesting question. A blockchain (at least one like Bitcoin) is an open, permissionless network.
What makes it 'more secure' is that each user is solely in control of, and are responsible for their
own private keys, which are a necessary and sufficient condition to perform transactions.