Agreed, I stopped recommending React to beginners. I still love it and think it's the future, but in the sense that it's the new low-level. Eventually I think it will be standard to compile futuristic, high level, built-for-apps languages into React code. But for now, makes sense to just use a batteries-included framework. I don't know what the most sure shot is there though. Ember? Vue?
Mithril. Much simpler than anything else and batteries included. It's a great way to get beginners into front-end frameworks and gain knowledge that transfers easily.
Mithril is nice. W.r.t. going for batteries included, it's still a bit minimal for my taste. Personally I would lean towards something more dogmatic, in the spirit of Rails. For example, things that I would miss from Ember: ember-data, models (with relations), controllers, opinions, integrated tooling, etc ... But Mithril looks cool I've been meaning to play with it too. You're right that skills gained from learning Mithril will be more widely applicable to general front-end dev than Ember.
Rails is great, but where it really shines is server side apps, content sites, blogs, CMSs, etc .. If you know you're building purely a SPA, Rails isn't really the right tool for that.
I'd argue that more than half pure SPAs do not need to be SPAs, and can be written effectively in Rails with someone who knows it well, in less time, with less bugs, and more test coverage.
