> IMHO, the main reason Rails is declining in relevance isn't microservices or the productivity (!) of Java, but the fact that more and more development effort for web applications is moving into JS front-end coding.

This. I also suspect that for developers whose first language - or primary language these days - is JavaScript it's a lot more comfortable to reach for a Node backend framework when they need it. Also, Go has become pretty popular for somewhat more custom or complex backend tasks. Plus, there are more and more high-level third-party services offering different pieces of backend functionality.