Currently, the monitoring checkbox is visible only when an operator is installed in a new namespace. We need to reconsider this limitation  - https://github.com/openshift/console/pull/5529#discussion_r430419501

It is true that there could be already some workloads running in the namespace, and potentially there could be an extra load on the monitoring stack.
However, a customer must explicitly opt-in for the monitoring during the operator installation (the monitoring checkbox is always disabled by default).

Probably there could be an extra warning message about potential consequences, but the monitoring should be allowed for existing namespaces. Currently, it is already problematic to get enough data from the telemeter (according to the research only a small % of installations opt-in for the monitoring), so by adding extra limitations it will be even more difficult to get the relevant data from the telemeter.