"But if most people use Warpcast, doesn't that mean it's de facto centralized?"

Depends on the perspective. 

For a developer, if you are building on the protocol, then you're free to sign up your own users or convince users to move over to your app (it's easy for users to use multiple clients at the same time).

For an account with an audience, let's say you were unfairly nerfed by Warpcast, i.e. you don't appear on the app. There's definitely a lot of friction to get people to start using another client if they already don't, but the protocol itself is not restricting your ability to communicate. That's an important distinction because on web2 / centralized social networks, the app and the "protocol" (i.e. the database) is the same thing.

Decentralization is indeed a key trend in the development of technology, including Lens. It offers more autonomy and security to users.