The second question is still a little tricky to answer, since you have no way of knowing for sure. You can only hope that if the original developers abandon it, that the community will step in, fork it, and keep the core of the framework intact.
However, we also don’t have a crystal ball, so while we consider recent activity, we can’t guarantee that any one of these won’t be usurped by a surprise challenger that nobody saw coming.
AngularJS was originally released in 2010, and is a web application framework primarily maintained by Google. It was originally developed to address challenges in developing single-page apps (SPAs), with components that complemented Apache Cordova, making cross-platform mobile app development possible.
In 2014, the original AngularJS team began working on an updated framework, which was released as Angular (or Angular 2+) in 2016. Unlike AngularJS, Angular is a single framework for all platforms, which is a big plus. Negative aspects of Angular include a much steeper learning curve than other frameworks — there are more than 30,000 “unanswered” questions on Stack Overflow, and uses non-standard HTML tags and attributes — and no compatibility between the two frameworks, so knowing AngularJS doesn’t mean you can also work with Angular.
Since react focuses on the “View” part of Model-View-Controller (MVC) development, other libraries and frameworks could be need for the “Model” and “Controller” parts. And while React is extremely popular, and backed by Facebook, it doesn’t mean it is easy to learn, particularly since, like Angular, it uses non-standard HTML tags and attributes.
With a typical size of just 20kb, Vue truly is a lightweight framework, that also promises to be simpler to get started with. The biggest risk is that Vue isn’t backed by a large technology brand, but the development team has grown from a single developer to a core team of around 20 developers, and many more contributors.
The developers also claim that Aurelia provides the cleanest and most standards-compliant component model you’ll find anywhere, focusing on ES2015+ and W3C Web Components. There is also an emphasis on patterns, making it easier to learn, combined with an effort to never make breaking changes to the API.
Web Components is not a new concept, nor is Polymer the only framework/library for Web Components. However, it does offer a simplified way for creating custom elements, along with support for custom gesture events, which work in both touch and mouse environments.
Ember.js follows a six-week release cycle, and the semantic versioning convention. All major new features are introduced early in the previous version, over multiple releases, helping to reduce the number of issues that usually follow a major update. While Ember.js is not necessarily an easy framework to learn, it does stick to common conventions and styles, making many aspects of developing easier. Something that warrants further monitoring is that — early in 2017 — Stack Overflow Jobs noticed a decrease in employees requiring Ember.js.
If you’re looking at expanding your skill set, you certainly won’t go wrong embracing React, Angular, or even Ember (despite its waning demand, according to Stack Overflow Jobs). Even though they include a steep learning curve, they are used by many big brands around the world, in a wide variety of projects. But newbies like Vue and Aurelia should not be overlooked, especially if you are looking for something lightweight, and almost modular in approach. Finally, if you’re looking at exploring a standards driven approach to Web Components, Polymer and Meteor offer that, along with the support of a stable primary developer.