You have mixed security problems with privacy and scalability problems, and bitcoin security at the blockchain level is completely different from the level of individuals and wallets
Security, scalability and decentralization are three components of
The Blockchain Trilemma. They interact with each other and affect each other, so I disagree if you say security has no relation with Scalability or decentralization of a blockchain.
Privacy and other issues like wallet security are different things, and they don't belong to Blockchain Trilemma components. It can be from a blockchain design, private or public ledger, a wallet software design and user practice too.