Frankly I'm not convinced of the value of this default behaviour - I think it's perfectly reasonable to expect users to add `ConditionMachineOf` for socket files (explicit is better than implicit), and as it is here, in the case where they happen to add a `ConditionMachineOf` for something else (or happen to set `Service=`) it could lead to surprise when it's no longer automatically peered with the right `.service`.