[untog]

Hm. I like the idea of gradual upgrades in theory, but I worry the reality will be sites loading at least two versions of React semi-permanently. And possibly even worse, you end up with an NPM dependency nightmare of off-the-shelf component X using an entirely separate React runtime.

Yet more excess JS downloading and running on client devices, all in the name of superior developer experience. I hope my cynicism is proven wrong!

[danabramov]

Totally hear your concern! To be clear though, it's not like we're forcing anyone to follow this approach. It was just broken before, in the same way that putting React inside a Vue/Svelte/jQuery app was subtly broken. So we've fixed the root cause but I totally agree it's only a solution to be used as a last resort. (E.g. like in our case, where many "long tail feature" components were written in 2013 and no teams own them.) In either case, even if you follow this approach, we strongly recommend to only load the second version lazily on screens that need it — like the demo shows.

It's very suboptimal but I think it's still better to have that option than not to have it — or to try it and run into insurmountable problems. Especially in big long-running projects like internal company dashboards where long-term maintenance is more important than time to first render.

(I don't know why you're being downvoted though! I think your concern makes a lot of sense, and we're also sensitive about how to balance the messaging so that people don't use this unnecessarily.)