Even if the Calendar API were open to all platforms, it's not a standard, it's just an API unilaterally defined by Google. CalDAV, however, is a standard <https://tools.ietf.org/html/rfc4791>. It is certainly an evil move from Google to try to replace an existing open standard with an API defined exclusively by them (unless they have technical grounds to do so, and considerate and documented extensions to the existing standard are not a viable approach).
To make things worse, Google's support for their own APIs is pretty weak - I had some bad experiences with them.
Calendar API wasn't the worst - okay, I didn't get any response from Google guys on a bug I ran into, but they eventually fixed it (and lack of user support is pretty much a norm for them now).
The situation with Spreadsheets is way worse - the API is pretty much abandoned, no changes or any reaction to bugs/complaints for a few months now. They'd roll out a new API version, remove the old documentation (instead of marking it as deprecated but accessible), but the new documentation is really lacking - only has examples in C#, and even these are incomplete.
So, yeah, moving from open standards to their own API is very bad - not just because it's locking up people or something, but also because they are not supporting _their own APIs_ properly.
Yeah, CalDAV != Calendar API. I've always had nightmares hooking anything to it reliably over time. I finally just gave up and use Googles stuff to access it (after years).
