It depends on your options for managing state. If you can build a largely stateless UI, that can update from the server via a form submission or query parameters, then it’s a big win, both in terms of your code complexity and the complexity of the team you need to build it. You can still have lots of widgets on your page and even fancy CSS transitions to make it feel like an SPA if you really want. But if you’re making something very highly interactive or canvas based, with complex stateful UI components and gestures, then you probably have nothing to gain.

Anywhere I could conceivably get away with HTMX over React I would (and tbh I’d launch with no JavaScript at all before that). But if you’ve got a massive React codebase then you’re already in quite deep, both codebase and team structure.

All that said, some really high level apps are moving to WASM anyway, so I don’t know that an investment in React is safe for the long term.