Sorry but I disagree. Just take any of your a bit bigger React projects and transform them to Next or the way around. In. One. Hour. And production-ready please. Good luck, man (this is vendor lock-in).
If your threshold for vendor lock-in is one hour, there's no point discussing this. But I could probably do it in a day or two, which is good enough in my mind.
I actually just did this a couple days ago (from Razzle to Next), took around two hours mostly fiddling with updated dependencies. Will take longer for a complex app but the lock-in is way weaker than traditional frameworks.