While I can't speak for mobile rewrites, many systems at Uber have been rewritten several times as the business needs have changed and we've grown in scale. With each iteration the systems become more general and support a greater scale and a greater variety of business needs. The added complexity of our micro-services architecture has been worthwhile because it's reduced the complexity of making changes to systems that can't be taken offline because they are part of the core trip flow. Basically, it's as if we started with a small glider years ago and have upgraded it into a 777 Dreamliner piece by piece while still flying.
We have also taken advantage of the rewrites to rebuild in Go and Java. Most of our older systems were NodeJS and Python. Many of those have been rewritten in the two languages we've now mostly standardized on.
Pretty much every rewrite I have witnessed has gone well, so I can only guess there's something we're doing right in this respect.
Lots of reasons such as performance, type safety, static analysis, etc. NodeJS worked well when we were smaller, but it's not uncommon to work in code you didn't write and Golang and Java are languages where the compiler helps you a lot.
One of the best policies we have at Uber is our internal transfer policy. So long as you're in good standing perf-wise, you can transfer to other teams and projects within 1-2 months. For such a policy to work, it needs to be easy to work in unfamiliar codebases. I can't speak for Java, but it's much easier to drop into an unfamiliar golang codebase and be productive and not introduce bugs than it is to do that with NodeJS or Python.
I do exclusively golang work now but when I joined I was doing exclusively NodeJS. While the first two to three months with golang were a bit frustrating, I can't imagine writing a backend in NodeJS ever again. That said, I would still use NodeJS to build frontend developer tools, but that's about it.
We have also taken advantage of the rewrites to rebuild in Go and Java. Most of our older systems were NodeJS and Python. Many of those have been rewritten in the two languages we've now mostly standardized on.
Pretty much every rewrite I have witnessed has gone well, so I can only guess there's something we're doing right in this respect.