The same goes for almost any legacy app.
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.
You throw away a lot of stuff for a rebuild.
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.
