The only way to get calendar files (.ics) from iCal calendar backups (.icbu) is to recover the back in Apple Calendar (iCal) and save the calendar data in ICS format. You can always check Apple Store for new apps, maybe some new appeared that can help you with this and automate this procedure.
Convert Apple iCal backup to iCalendar calendar file format (Microsoft Outlook, Apple iCal).
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert icbu to ics on OS X
Apple Calendar (iCal) is a personal calendar application bundled with Apple Mac OS X since 2002. Calendar was the first calendar application for Mac OS X to offer support for multiple calendars and the ability to publish/subscribe calendars to WebDAV server.
Convert from icbu file
to ics file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.