I can't speak for him, but upgrading really old Rails apps can get complicated very quickly. Especially when you're going across multiple major versions and have to deal with significant changes in Rails behavior and broken gems. "Rebuild" might not be the most accurate way to describe the slow, steady incremental approach you're forced to take (you aren't redoing huge swaths of your domain logic, for instance), but it gets the gist across.
I've been working on a .net web app, that's been around since 2008. It's been continually evolved, so it's running on the latest MVC framework, uses microservices etc
As result it's build up a huge amount of automated tests. The business logic has been built up from experience and is well tested even for odd cases.
Oh, definitely. It's not something I'd recommend ever doing for the hell of it unless you're a real masochist. Even then, it might be easier if just broke out the floggers.
All kidding aside, if the concern is just for continued security patches, there's always Rails LTS [0]. You have to pay to play, but it's cheaper than a security breach or weeks of development time. But if you're dealing with significant performance issues or looking at major feature changes/additions, it might be more effective in the long-term to consider an upgrade. You just need to be aware of just how large a project that can turn out to be.
Long story... we had the git repo but lost access to any working installation. I had to rebuild a dev vagrant VM first, and later on a production-ish setup on a server.
