- Angular had the spot, then bumbled the transition from 1->2->5.
- Durandal died/was transformed into Aurelia, whose hype never materialized.
- Backbone +/- Marionette seems to be in super slow decline, but doesn't update as fast as Ember.
- I don't know much about Polymer but it just seems like an interesting project, not something I'd suggest an org to use (I'm probably wrong).
Ember shouldn't be compared to tools like React/Vue because it's almost never just React/Vue, and Ember itself has lots of plugins that come into play. "React" =~ React + React-Router + Redux/Reflux/Fluxxor/Flux/whatever, and roughly the same with Vue -- though you can choose to extend Ember with plugins, by default everything you need is baked in, with ONE set of well-written if not long documentation. I can certainly say that Ember is more coherently put together than react/vue + react-router/vue-router + redux/vuex.
One of the biggest problems with ember of course, is the breakneck pace of change. I think it's slowed down somewhat recently, but keeping up with it can feel like a full time job sometimes, if you get stuck behind for too long, it gets harder and harder to update your codebase.
As far as I'm concerned, Ember should keep doing what it's doing, it's basically the only worthwhile competitor these days in the most-batteries-included (aka frontend framework rather than frontend library) space. If anything these days, Angular (5?) is looking like it's pivoted it's way to essentially emulating Ember, so I'd place it second.