It's also worth noting that there's a new Vulkan compatability layer for Metal -- so if OpenGL did go away for some reason, at least there's Vulkan to replace it (finally).
Not quite. MoltenVK does not possess either tessellation or geometry shader support yet. As such, it's usefulness really only extends to D3D9 API level games and the OpenGL equivalents. And because Metal does not yet have geometry shader support, the lack of said functionality means that even if MoltenVK got that functionality, it still couldn't act as a wrapper because there would be no equivalent in Metal yet. Metal just got tessellation in 10.13, which was finally enough to port Overwatch to OS X (which won't happen - too little, too late for Blizzard; thanks Apple). The best case scenario we'll see in the near term for Vulkan on OS X is MoltenVK allowing for a port of Doom 2016 to OS X. The more likely reality is that it'll be relegated to porting over a few of the more prominent but less demanding games on Steam.
There's no reason to compile Cocoa as 32-bit, and they want to get the Carbon overhead out of the OS. That makes sense to me.
a few exceptions to that, such as medical software. The FDA here has some very
funky certification regulations in place. For instance, the CT scanner in my dentist's building has certification only with a specific software package, which in turn is also only certified on Windows XP or OS X as a 32-bit app (i-CAT software). This is an issue we're running into because we have a Windows 7 Enterprise volume license, but can't update due to recertification requirements and the obvious testing that would be necessary. Of course, the number of Macs in use in the medical field is probably small enough for Apple to just jettison that tiny sliver of users with some legitimacy, though anyone that has machines connected to the internet for any reason will be required to update to current operating systems for security reasons or be forced to replace said systems.
The above is a real fun mess for my dentist since the CT scanner's software got it panties in a bunch and corrupted its configuration file, the issuing company has no way to fix it, and he can't get scans done (mine included) without it working and the simple route of just paying for a new software suite isn't an option.
As for Carbon, I thought they had finally jettisoned that long ago, but then I realized that your emulators still had some traces of Carbon left in them (I think), which is obviously why you're in need of redoing them to keep them working (and it gives you the opportunity to jettison excess baggage from its UI control code at the same time, which is nice!).