Angular 1.x still works. Even Angular < 1.2 still works and packs a powerful punch for less than a hundred kilobytes; it's more backwards compatible too. By the time Angular 1.4 came out the framework had grown to nearly one hundred and fifty kilobytes, and dropped support for some older browsers. Angular 2 is now in beta and it is over half a megabyte, and more or less requires developers to use language features not yet supported by current browsers.
So why would anybody want to start using Angular 2, let alone migrate to Angular 2?
Advantages of Angular 2
Angular 2 is Easier
Years of feedback have made Angular 2 more modern, more capable, and easier for new developers to learn than Angular 1.x. In turn Angular 2 is also easier for project veterans to reason about. Ultimately this makes for a codebase that's all around easier for everyone to deal with.
Angular 2 replaces directives and controllers with components; think of them as refined directives. Even experienced Angular developers are not always aware of all the capabilities of Angular 1.x directives. Angular 2 components are considerably easier to read and their API features less jargon than Angular 1.x's directives. Additionally to help ease the transition to Angular 2, the Angular team has added a .component method to Angular 1.5.
Despite being a complete rewrite, Angular 2 still shares many of the same core concepts and conventions as Angular 1.x; like a streamlined and "native JS" implementation of dependency injection. This means that developers who are already proficient with Angular will have an easier time migrating to Angular 2 than another Framework, like React or Ember.
Performance and Mobile
Mobile use of the web is huge, and growing. Mobile devices keep getting more powerful, but they're still quite underpowered when compared to a modern desktop; mobiles typically have less data bandwidth to work with.
Angular 2 was designed for mobile from the ground up. Aside from limited processing power, mobile devices have other features and limitations that separate them from traditional computers. Touch interfaces, limited screen real estate, and mobile hardware have all been considered in Angular 2.
The nature of computing performance means that desktops will also see dramatic improvements in performance and responsiveness. Early benchmarks performed on Angular 2 alpha confirm that "dramatic" is an accurate choice of word. The author makes the wise disclaimer that benchmarks are tricky to get right, and even when done correctly can be misleading. Nevertheless, Angular 2 alpha's rendering performance vs Angular 1.4's is astounding at scale.
Like React and other modern frameworks, Angular 2 can leverage performance gains by rendering HTML on the server, or even in a web worker. Depending on application/site design this isomorphic rendering can make a user's experience feel even more instantaneous.
Project Architecture and Maintenance
The first iteration of Angular provided web developers with a highly flexible framework for developing applications. This was a dramatic shift for many web developers, and while the framework was helpful, it became evident that it was often too flexible. Over time, best practices evolved, and a community-driven structure was endorsed.
Angular 2 is built on years of user experience with Angular 1.x and offers many improvements. However there are still two advantages to Angular 1.x:
- Developers are familiar with it
- Angular 1.x is not in beta
Some new projects that have tight deadlines, and/or limited development resources would benefit from being written in Angular 1.x, using an Angular 2 style. This would allow for a straightforward upgrade in the future while making efficient short term use of resources.