So I'm aware of at least BBC using this approach, where opening really old articles reveals they have their 90s site still up. I'm also aware of at my last employer, an unmaintained wiki with open security issues that nonetheless had vital information for still in use legacy internal software was replaced by saved static HTML grabs so the information wasn't lost.

But for a lot of medium sized companies with dynamic websites, this isn't always practical. They may not have the know how to dump their 2000s drupal install to static HTML files, and don't have the IT staff to upgrade and secure it.