There's really a need to sync the calendar with further providers other than Google and Microsoft. How come such a numerous amount of Apple users are able to sync directly the calendar? And the CalDAV protocol is quite extended.
You're forcing us to use Google, who already knows too much about any of us. There's really no need for them to have an eye on all the business meetings that we have.
I've already forwarded this request to the development team over 1 year ago. Please make it happen!