In 2015 my team had Backbone, Angular, and React in a single app for a period of time. We merged two separate applications (Backbone and React) from two separate teams into a single application. We gradually moved the application over to entirely React over the course of about 7 months. not that uncommon.

> not that uncommon.

That is my understanding as well, it is especially common to see this when a product is migrating from one framework to another. A quick google search turned up plenty of examples: http://tiny.cc/jmur6y, including Microsoft's tool specifically for this purpose: https://github.com/microsoft/angular-react.

Angular 1.x has this dreadful thing called digest cycle. Long and short of it, the rendering performance degrades proportionally with the number of things Angular has to watch for changes.

Some people retrofitted busier parts of their apps with react views to work around that. There are even a frw angular directives for that out there.

At my previous job we started out with Angular in 2017. After some time passed benefits of using React was clear to everyone in the team. We wrote new features in React. The old Angular code worked as expected. It made little sense to spend time porting that to React.