I think you are right. plasma-systemmonitor is already a part of the plasma-meta package group and is meant to be a replacement of ksysguard. Better if you create a issue here:
Note - Changed title tag from “Support” → “Feedback”
Plasma System Monitor App is the successor to KSysGuard, but if you have the plasma5-applet-thermal-monitor package installed, is still dependent on the KSysGuard …
No need to create a issue for something they are already aware of. Just let the Plasma devs remove Ksysguard over time as they add the few remaining feature from it to Systemmonitor.