If this is a limitation, it's a limitation of Akonadi that it doesn't monitor remote single files for changes. A workaround could be to use the local file source, let korganizer monitor it for changes, and sync the local file using wget/curl and a systemd --user timer. This should work flawlessly and be solid.
Personally, I'd suggest to think differently and get the ICS subscription into your DAV calendaring system first. CalDAV software or hosting worth its money should support some kind of calendar subscription. This is exactly what I'm doing; I have CalDAV as the single source of truth, and subscriptions of external calendars flow into the DAV calendars. At the endpoint device, I just need to connect to CalDAV to get everything from one source. This is also the setup I'm using with KOrganizer.
