React is what 3-4yo now... given Redux is less than 2 years, and there was a LOT of churn on flux-like frameworks. But React + Redux will likely not go anywhere for the next 5+ years, it's already pretty well distilled. Similar tooling and alternatives will still be present, but this happens in any vibrant ecosystem. There was a similar burst in the alt.Net space before ASP.Net MVC and razor came out... even then there has been alternatives introduced.

You're not talking about a single application and platform. You're talking about the most flexible set of cross platform rendering engines ever created. How many UI toolkits are there for Windows, Linux and macOS for native apps, now add Android and iOS... The browser targets all of them, and the base app toolkits pretty much target them all, and still being more flexible and capable than what came before. Expand this to the number of tools available. How could this be anything but echoed in the JS sphere.

Given the shear breadth of Web development alone, let alone server-side, IoT, Desktop, embedded, mobile and who knows where else, how can there be anything but a lot of options and diversity.

That is not true at all. It's also not an issue that is going away. This is The Singularity. It's only going to get worse. Either you learn to adapt or you get left behind. And "adaoting" could mean, "learn to quit being a magpie distracted by evwry shiny new thing."

This seems to be true, but I see it as a consequence of how many resources are being devoted to JavaScript by major corporations and legions of developers versus other programming languages. Does Google, MS, Facebook, Apple, etc. ad naseum have entire departments devoted to improving Python, C, Ruby, etc.? Not as far as I know. If there were as many resources being thrown at C as there is at JavaScript I'm sure the churn would be just as high.