The browser is becoming more of a platform to deploy to like Android or iOS, instead of a document viewer. I think it is still the wild wild west on the client side for now, but once winners emerge there will be a much larger group of developers creating SPA's. It seems Angular 2.0, Ember 2.0, and React 1.0 are all headed in similar directions with their emphasis on components, so a winning concept might emerge some time soon. That said, development trends on the web slowly play out over years. I expect Rails and Django to have a massive user base for 5+ years at least.