Traditionally, even for games consoles, platform holders are not usually motivated by companies delivering ports that are identical to those from other platforms. They really want exclusive products for their platform or at least have the superior version on their devices.
Whilst this makes sense, surely having a popular title that runs identically to its counterpart on another platform, is better than having nothing at all. And let's face it, Apple won't get games that run better on macOS than on Windows because of Metal... And for the tripple A market, not exclusives either. The developers will make it with Windows in mind, and the GPU vendors will do driver optimisation for the Windows drivers to work with the game. That won't happen for macOS.
Arguably, they might be able to take some performance wins with creative software with Metal, and Final Cut already beats Premiere performance wise by a long shot, but again, Windows is the dominant platform on the desktop, so I refer to your second paragraph.
Correct, the dominant platform holder is in a very strong position, so developers tend to optimise or release only for their platforms - see the NES & PS1 for examples.
On iOS, Metal only makes sense with respect to the above. But with the Mac, Vulkan as an alternative could draw some of the software that would otherwise not be produced for the Mac to the platform. The Mac isn't the dominant platform in the desktop space, and only getting iOS game ports to the Mac with Metal won't exactly make the Mac look like a serious desktop. And I'm not saying that's all there'll be, as you've proven earlier, UE4 is a great example of serious Metal use and such, and Metal seems fine considering it's current maturity level, but for the not-dominant platform, betting on cross-platform technologies seems like a good way to attract more software.