Customer reported and I was able to duplicate the issue. The altersystem-disablemodule command allowed the user to disable the only UM in the system. This shouldn't be allowed, whether its a UM or a PM.

Component Status Last Status Change
------------ -------------------------- ------------------------
System ACTIVE Tue Feb 26 15:41:42 2019

Module um1 ACTIVE Tue Feb 26 15:41:40 2019
Module pm1 ACTIVE Tue Feb 26 15:41:31 2019
Module pm2 ACTIVE Tue Feb 26 15:41:32 2019
Module pm3 ACTIVE Tue Feb 26 15:41:33 2019

Active Parent OAM Performance Module is 'pm1'
Local Query Feature is enabled
MariaDB ColumnStore Replication Feature is enabled
MariaDB ColumnStore set for Distributed Install

mcsadmin> altersystem-disablemodule um1
altersystem-disablemodule Tue Feb 26 15:42:19 2019

This command stops the processing of applications on the Primary User Module, which is where DDL/DML are performed
If there is another module that can be changed to a new Primary User Module, this will be done
Do you want to proceed: (y or n) [n]: y

Stopping Modules
Successful stop of Modules

Disabling Modules
Successful disable of Modules