There are some problematic users misuse and/or abuse the minor edit flag (e.g. to try and hide vandalism or controversial changes). Preventing them from marking edits as minor would, in many cases, be a more proportionate response to the problem..
It would be useful to be able to *only* block users from marking specific namespace edits as minor.
In fact, this is very similar to T274911, But this task is limited to a specific namespace.
For example, there is a user who only edits in the template namespace and the main namespace. The user's edits in the main namespace are mostly minor syntactic changes, so marking them as minor edits is not a problem. However, this user defaces template namespaces and uses minor edits for all destructive edits. Therefore, the administrator should block this user from editing template namespace or block it to use minor edits on template namespace.
The next thing that your fictional user might do after editing an existing page and not being able to edit an existing page anymore might be creating a new page.
@IN: I do not think that the gain outweighs the costs here. Adding any additional code (and having to adjust the user interface, and having to update documentation) which covers more and more rather pretty uncommon corner cases creates maintenance costs (testing that code, adjusting that code) for many years to come. Please check https://www.mediawiki.org/wiki/Bug_management/Development_prioritization . I have boldly declined this request.