The majority of the complexity is in the ancillary tools and ecosystem extras, which are not strictly necessary. You don't need bundlers and transpilers, for example. The only reason they are popular is because JSX is a good convenience, though.

You can get away with very basic React for the majority of cases. Things like advanced lifecycle shenanigans are also not necessary, unless you’re doing something extra-smart, or inherently complex (due to browser APIs, third-party APIs, or just complex design patterns). But those are also not necessary: just nip complexity in the bud if possible, but if something becomes impossible to avoid, the complex side of React will have your back.

The main difference between jQuery and React-the-library is that React handles a lot of the complexity around state and component abstraction all by itself.

With jQuery the complexity and spaghetti appears much earlier than in React. It's even difficult to compare, it is much simpler to keep a React codebase cleaner than a jQuery one.

Of course, if your app is simple enough, you don't need the "extra complexity" brought in by React, and jQuery will serve you well.

But this is also true even for "simpler things" like jQuery and Ajax: if your app is simple enough, you don't need Javascript at all, you can handle 100% with server rendering. And so on.