All these arguments apply to email as well - there are plenty of small providers and you can run your own email server. But, in practice, almost everybody uses gmail or outlook so we still say it's heavily centralized.

What good is running your own full Ethereum node if OpenSea blocks the NFT you're trying to sell and most of the customers who would want to buy it are going through OpenSea's node?

This is the exactly how the current web works and what web 3 is trying to change, regardless of opensea.