You never just want to "pull in external data" in a real app though. You need to make the request, deal with injecting auth tokens or headers, handle errors, handle different responses, update global and local state, possibly trigger side effects when the loading starts and ends, do something in a "finally" callback... If your app is non-trivial then you'll almost always end up using a library of some sort. That's where useSyncExternalStore is useful. If you're using useEffect then you're probably not making external requests well.
If you don't need it then you should be questioning whether you even need React, not whether or not you need useEffect.
I do agree that better built-in methods should exist that does most of what people have been using useEffect for. Many frameworks come with many tools for specific use cases but react has always left it to the community. If this or new ones are helpful enough, then I’m up to use them. However, your last statement is a bit too presumptuous to assume that not needing useSyncExternalStore means that someone does not need React. I recommend Remix for most apps now if possible, and the number of times anyone needs to use either hooks discussed is very minimal. Remix uses React.
Think your last reply overlaps with the first half of my message. I reach for swr or similar to manage the lifecycle of client side network requests (or any async requests) if necessary. I would imagine that these libraries or new ones adopt useSyncExternalStore internally. If I want to use this hook myself, it seems quite a bit more work to have to write a subscription-based store just to make certain network requests. Many apps aren’t real time either.
Take a single case: analytics. I want it to trigger whenever the page loads (and can’t be done server side for whatever reason). Are you saying that I need the kitchen sink to make this fire-and-forget request? useEffect is fine here. Am I not making an external request well? I just read the useSyncExternalStore RFC. Its primary purpose is to handle external state change in a concurrent environment. Makes sense. Is it to become the default for all “external requests?” Idk. But let’s agree that not all requests require the same care.
If you don't need it then you should be questioning whether you even need React, not whether or not you need useEffect.