Wow, does this mean Maya, Houdini, and basically every 3D package out there will no longer run on macOS? If so that seriously sucks for 3D pros.
I'm guessing you haven't used apps like Maya, Nuke, or Houdini. They were all written in the mid-90's on IRIX machines and later ported to Linux, Windows, and OSX. Surprisingly, 3d performance isn't always big goal of their's. My guess is the core features don't sell new versions, so even though they have annual releases those things don't get much attention. They'll have drawing issues and transparency sorting problems for years. Same with audio bugs.
Their Mac support was spotty and irregular until the past 5-10 years.
 at least the portion I follow on twitter
Metal and OpenGL two completely different APIs, shading languages and probably a whole host of other things.
I've ported my fair share of things from fixed-function to programmable shader pipelines and you'd be effing naive if you think you can do that in a couple weeks on anything more than a toy demo.
Getting pixels on the screen and shipping something to end users are to very, very different things, 90/10 rule and all that.
Vulkan also has the benefit of multiple platforms supporting it so you're not doing all that work for a minority(which is what OSX is in the graphics space) platform.
I develop an OpenGL-based video engine for a live media playback application, which is very nearly as simple an application of OpenGL as you can expect to find in the real world, and there's no way I could expect to port it to Metal in a week or two singlehandedly. Like others have mentioned, it's a completely different paradigm, not just a matter of changing around some function calls.
That said, I welcome this change with open arms (and secure in the knowledge that legacy code will continue to work for the foreseeable future). OpenGL is a fragmented, brittle, spaghetti-inducing pile of global state. Rewriting in Metal isn't anywhere near as small a project as Apple claims, but I'm perversely looking forward to it -- I'll be very happy to have OpenGL in my rearview mirror.