No I don't think DeFi is a threat to centralized finance because not all DeFi projects practice what they preach, they claimed they are decentralized but they practice centralized strategies, most DeFi projects team or developers don't even understand what true decentralized is.
The difference between DeFi and the existing financial system is that all financial services are recorded in smart contracts that cannot be changed from outside. However, there are risks of using DeFi, since in case of an error in the smart contract, participants may lose their money. With centralized funding for all of the errors can easily be corrected by the employee.