Comment by jlarocco
I don't think your understanding is correct.
In the 60s, 70s, and 80s it was more true, but nowadays Catia, NX, Creo, and the other big CAD systems are monolithic applications with extensions and add-ons (either from the vendor directly, sold as third party add-ons, or developed in house with CAA/NXOpen/ProToolkit) for doing stuff like composite design, sheet metal, various types of analysis, and a million other things.
Under the hood they're very modular with hundreds (or thousands) of shared library modules, and sometimes those modules have long histories going back to the 70s, but the UI of the 70s is long gone, and they're used like normal shared libraries, not piping around data on the shell.
Not that it doesn't happen - I'm sure some companies have old tools they haven't updated in decades - but the day to day CAD work is all GUI based.
That's my impression working at a company doing CAD translation, anyway.
In a lot of large deployments, the CAD data isn't even stored in files, but in a PLM database. For example PTC's WindChill and Dassault's Enovia/3DX.
Correct me if I'm wrong but I think PLM/PDM are storing the CAD model as "CAD databases" (basically the proprietary file-format) in a Data-Vault. So while not all CAD data is being stored in those files (meta-data for example) those old file-fomats are still in use I think.