Excuse my ignorance, but how is that migration (especially of older libraries that are apparently being rewritten) not just a copy/paste action from one server to the other? When I build software to deploy it it includes everything it requires library wise. At least the few things I've deployed so far.

You have to copy data across, and confirm that everything worked correctly, and if you're being fancy about it you need to freeze writes to the old server while you are migrating and then unfreeze after you've directed traffic to the new server. It's not trivial.

Sometimes you need library version X, which uses a compiled binary for the platform, which requires C library version Y, which requires glibc version Z, which is deprecated on the current version of the OS, etc etc etc.

Or you can update the app to remove the dependency on the library.

But honestly, this is what containers or VMs are built for in the first place.