Preventing Qalendar from being terminated on D-Bus activation timeout is as easy as registering a dummy service (one line of code). The real problem is that when the application is closed, the service is unregistered and the calendar is automatically restarted. I have no idea how to stop that.

Disabling the respawning can be the best solution because of simplicity (?) and resource usage, but as I have already mentioned a few times, I am afraid that there is some reason why Calendar is running in the background and mediaplayer or worldclock are not.

IIRC, even a hello world in Qt can take over 30 MB of memory, but the libraries are shared, so 2 Qt applications could take for example 35 MB total. But I guess that's still bad if Qt is not used in any of the startup applications.

Adding an option to run Qalendar in background would be only the first quick step, just to check if there are any problems after dumping the default calendar (stopping the respawning could serve that purpose as well, but I wouldn't be surprised if it is hardcoded somewhere). Later it could be replaced by some kind of non-Qt stub which would launch the real thing when required.