This is to ensure that files that are uploaded, and any templates transcluded in the MediaWiki namespace, are automatically protected by cascade protection.
This has nothing at all to do with the action API, removing MediaWiki-API.
I note this would likely cause people with the 'editinterface' right but not the 'protect' right to be unable to edit anything so protected.
I also note this probably wouldn't work as you intend if the files/templates are only used in the message based on the message parameters ($1 and so on). And also won't work for messages that haven't been customized, but that's very seldom a problem since the default messages usually don't use templates or files.
Hi @UpsandDowns1234, thanks for taking the time to report this!
Could you please edit the task description and elaborate by adding a more complete description to this report by elaborating on the currently needed steps for each of the situation you mention and their underlying problems, and why those situations lead you to propose this solution "Cascade protect the entirety of the MW namespace"?
Helping others to understand the underlying problem might lead to better solutions. Thanks!