I think the important part of decentralization is not that “everyone must” own their own server, but rather that “anyone can” run their own server, that indexes

I think the important part of decentralization is not that “everyone must” own their own server, but rather that “anyone can” run their own server, that indexes the globally consistent blockchain database. But, I agree that the most troublesome parts are around the client/server relationship due to the need for indexing/caching, and the irony of having a man-in-the-middle between you and the trustless network.