We are looking for a way to cope with AngularJS' LTS coming to its end. We have been working for about the last 3 years on a project, written using ASP.NET Core + AngularJS 1.5.7 + Fuse theme. Altogether we've invested about 10 development years into it. The alternatives we've looked at so far were migrating to Angular2+ or React, but we expect those alternatives to be too costly (we can't afford ourselves investing ~3 development years on such a migration project). We've also considered side-by-side development – that is leaving the current code as is, and using some more advanced technology only for new features – but we couldn't find a reasonable way to implement it, and it won't actually solve the main issue we have with the LTS ending. The main issue for us with AngularJS' LTS coming to its end is the lack of security updates that come as parts of new releases (and of course we are happy to advance to a newer technology regardless).
So which technology is the simplest to migrate to, and is there some tool or library that can help us?
I'd recommend React. This is because React is very easy to learn. Furthermore, you get to code your dynamic UI components like classes and objects which will make them reusable all through your project. I find that as being one of the best features React has. Also, you get speedy development with React due to its simplistic syntax and the sharing of data among components through props. Also it's backed by Facebook and the framework is easy to integrate with other applications.
I did this migration in 2016-2017 on a pretty large app. Search "migrating from angularjs to angular" and you'll find tons of advice. And some tools. We chose to upgrade based on route, where basically more of our routes were handled by Angular over time and less by AngularJS. The whole migration took about 18 months, the app continued to work fine, and eventually, it was all migrated.
Here are some tools that can help suggest a path forward. https://blog.angular.io/migrating-to-angular-fc9618d6fb04