We have used Angular JS extensively in our company for enterprise projects, mobile applications and continue to use it. Google's decision to stop development of AngularJS and put it on a EOL will definitely make developers to panic. But 3 years to migrate away from it is a long time and you can plan accordingly.
If your plan is to migrate to Angular, then you can follow their official guide to upgrade using ngUpgrade. You can find numerous articles online that explain how they upgraded existing AngularJS apps to Angular.
If your plan is to consider migrating to a totally new framework, then this will involve some work. You should take a look Web Components spec. Your existing directives/components can be re-written, with less effort, as web components (shadow DOM) or custom components (without shadow DOM). There are libraries that help you write these generic components - supported on most browsers today - Stencil JS, lit element and a few others.
The advantage of using Stencil JS is that it provides tools to compile your web components to target different frameworks (Angular, React, Vue, Ember).
The latter solution seems feasible as it allows you to migrate directives one by one over a period of time, without having to re-write the whole application in one go. In the future you can also re-use your components in the framework of your choice.
As for your question about will it continue to work after 2021 - yes it will continue to work. The problems you might face might not really be of technical nature, but related to hiring resources to work on it or maintain it.