I've been thinking about this a lot over the past year. If Apple announces nothing at WWDC this year Apple is going to be in trouble for OS X. Apple needs to announce one of three things at WWDC or OS X is going to be severely behind the curve on having a decent 3D API. Vulkan support (best option IMO), metal OS X implementation, or at the very least kick the can by adopting OpenGL 4.5 support.
The first, and in my opinion best option is for Apple to be an early adopter the new Vulkan API. Others are 100% correct that OpenGL is used for many professional apps and OS X itself. Keeping the API in the OS is going to be important for backwards compatibility and portability. Updating some of OSX's high-end APIs (Quartz, Core Animation) to Vulkan could really be a lot more performant. Vulkan seems to be a complete cross-platform Metal replacement. With gpu compute, shader compilation, and a low-overhead graphics API Vulkan would be a pretty close feature comparison to Metal. The problem with Vulkan is that it is very late to be ready for OS 10.11. They just announced a preliminary spec at GDC in March, and Apple has been very slow to adopt the new OpenGL standards. Sure some of the heaviest backers have some alpha version drivers, but the group is full of a lot of different views and it takes them time to come to any cohesive decision. I'm pretty sure Apple sees this as a liability as Apple doesn't like to be held back by companies not in their control. They may view the lagging OpenGL group as a means to cut them out of the equation.
In the meantime there is Metal and the possible porting of this API to OS X. However, there are problems porting Metal to OS X. For example the current API is very iOS focused. The API assumes a unified memory architecture (same ram for both CPU and GPU). Metal also doesn't support some important DirectX 11 features like tessellation. A problem with Metal is that while it is a decent API, I have worries about Apple being able to dedicate the resource keep pace in the rapidly developing 3D graphics industry. Leaving features that are available on other platforms out of the API makes it problematic to port or keep feature parity with on OS X versions of the software. Let's be honest, OS X does not have the marketshare that iOS does on the smartphone market, I don't know how much pushing of new APIs they can really do on the PC devs.
Then there is the lame approach of just updating to OpenGL 4.5 or keeping OS X at 4.1 feature set. This I feel is a disaster due to ignoring the problem for another year. The future is Vulkan and pretty much everyone in the consortium and industry agrees. There are already features that even Linux has, that OS X doesn't. That's right, thanks to the help of companies like Valve, Linux is becoming a better gaming OS than OS X. Apple is a forward thinking company, and they have no doubt thought about what direction they will go. However, if they just stick with OpenGL proper for a year I am going to be very disappointed.
Anyways, I REALLY hope that Apple announces their future 3D API with 10.11 at WWDC. Only a couple months away! Here's hoping Apple being a tier 1 supporter of Vulkan or a new OS X metal that has feature parity with D11 or D12.