App Engine didn't support chunked transfer encoding so progressive data loading becomes impossible, and Angular v1 is a pathological performance and maintenance nightmare to the point where AngularJS / Angular v2 ended up being an incompatible rewrite.
The problem is, if you have hundreds of people contributing to a tool with deadlines to meet and lots of UI components you need an incremental approach if there's going to be any hope of replacing the whole architecture. Without an incremental path forward, rearchitecting is dead in the water. Management isn't going to want hundreds of people stop working on new features for months while the whole UI gets written from scratch.