Seems unlikely they'll drop it given their approach to iOS so far has been to infiltrate the ecosystem with their own Google apps, though never say never I suppose.
As for why they're pushing people towards their own Calendar API, presumably so they can add/promote new features specific to Calendar which aren't present in CalDav. They can get more usage of Calendar-specific APIS/Features by pushing devs towards their own version, instead of letting them default to the CalDav API because that's what they're familiar with.
The responses gathered from API request form also act as a feedback mechanism for their own Calendar API, which they can use to fix any deficiencies.