Meanwhile, I discover that there is a module in kde called kephal which is supposed to poll for xrandr changes.
Somehow, it got turned off. (it doesn't work with nvidia drivers but it should work with intel hardware)
polling is controlled in ~/.local/screen-configurations.xml
and can also be set with
qdbus org.kde.kded /modules/kephal/Configurations setPolling true
which apparently polls every 10 seconds (no configuration possible for the period of polling).