Organizations with large websites use content management systems. Vendors of content management systems come and go as technology changes. Usually, changing the CMS a website uses necessitates URI structure changes depending on the conventions of the system.
Maintaining in perpetuity a complete set of redirects (or stack of rewrite rules) for every page for a website consisting of millions of pages for decades is not feasible in most cases. Things change, departments are created or disbanded or renamed. Management decides certain things should not be accessible to the public or stored in a particular location.
It's incredibly unrealistic to expect every URI to be permanent.
No, I'm saying that technology changes, the web changes, your organization changes and the content on the website changes. To act like it's even desirable to have all of the same documents at the same locations on your website in 20 years is misguided.
Maintaining in perpetuity a complete set of redirects (or stack of rewrite rules) for every page for a website consisting of millions of pages for decades is not feasible in most cases. Things change, departments are created or disbanded or renamed. Management decides certain things should not be accessible to the public or stored in a particular location.
It's incredibly unrealistic to expect every URI to be permanent.