As I have elaborated e.g. in T196215#4257219, Monobook is only used by experienced editors who prefer it because it provides them with a consistent working environment. There is no need for a responsive Monobook for this user group.
The (first) activation of the responsive Monobook skin caused significant, negative responses, with user scripts breaking the site layout in the mobile view (T196215).
I would therefore like to request that the user setting to enable responsive Monobook is set to disabled by default. If necessary, we are happy to demonstrate editor consensus.