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> I [...] think it's more likely that some part of the Web Components tech stack will be removed from Chrome, Safari, and Firefox in the next 20 years, consequently breaking apps built with them, than JavaScript will change in a way that means a React 16 app I wrote 2 years ago will break.

Hmm. I think you've specified a bet you are likely to win, but one that's nearly meaningless. Perhaps if you fully lock down all dependency versions your React 16 app will not break, but that's a little more like running an old program in a VM or an emulator than "I can still work and hack on my React 16 app."

A more meaningful bet would be, "can you still develop on a React ZZZ app vs. can you still develop on an app built with web components."

If I got to pick the terms of the bet (to favor my viewpoint XD but also to make it more meaningful), they'd be "would updating a React app to catch up on N years of dependency updates be more or less effort than updating a web-components built app to catch up on N years of dependency updates"; if you keep your web-components built app from having a bunch of weird dependencies, I think the odds are very much in favor of the React app taking more effort to update.

Basically, betting on browser vendors to be less likely to break backwards compatibility than the authors of React.