Ethereum gets criticized a lot for "overinvesting" in security and decentralization. But one way to make sense of this "overinvestment" is to view Ethereum as "the system of systems."
Ethereum wants to run the world's financial system. In the future, it may also run the world's identity system, or even governance. And within these mega systems, there are smaller systems such as DeFi protocols and DAOs. In short, Ethereum is a system that hosts other systems.
What makes a system good for hosting other systems? Let's start with the observation that it's really hard to build a secure system on top of an insecure one. You can develop the most secure DeFi protocol, formally verify it and everything, but if the underlying chain can be easily attacked and re-orged, there's really no meaningful security guarantees you can offer to your users.
Similarly with permissionlessness. Imagine you built a totally permissionless DeFi protocol that anyone can use. Well, it can really only be used by anyone who can access the underlying chain. So if the underlying chain is permissioned or can be easily censored, it doesn't matter how permissionless your protocol is.
On the other hand, if the underlying chain is permissionless, you are free to build more permissioned protocols on top -- just enforce permissions with your smart contracts. That's why enterprises are able to build on Ethereum in practice, despite needing to enforce permissions.
Therefore, a good "system of systems" is one that provides an extremely high degree of security and permissionlessness, so that the systems on top don't get compromised through the underlying system, and so that they can be as permissionless (or permissioned) as they want to be.
Ethereum will host the world's most important systems. That's why it needs to be extremely secure, decentralized, and permissionless.