plasma-systemmonitor doesn't have any tabs, like ksysguard has. This doesn't fit with other other KDE applications such as dolphin, kate, okular, konversation, present ksysguard ... Can we please have tabs in plasma-systemmonitor?
The decision to use a sidebar is based on this discussion: https://phabricator.kde.org/T11153 . The recommendation here is to use a sidebar for navigating applications that do not have a changing list of pages. While you can change the pages for system monitor, I do not consider that a primary interface element and for many use cases, the list of pages is a static list.