OS coverage vs pure numbers is something for the game developer to be worried about, and by extension the game engine devs.
You want to get into the iOS gaming market, go Metal. By definition you've expressed you don't care about the other OS's.
You want to make a AAA game, you probably don't go Metal if you want enough market coverage to recoup your costs - though I will be delighted to be proven wrong when a AAA iPhone game comes.
If Apple want to change their "AAA game" attractiveness they need to address this. It's not that hard - the article took a month but even a year is fine. Apple just needs to get it done, but it doesn't seem like they care to make User<->GameDev relationship easy. Either the gamedev needs a whole other version of their game on Metal. Or the users have to do a lot of homework and fiddling to get a modern AAA game to run.
I wonder if they have Vulkan internally and simply know their GPU won't look good in direct comparisons - which is fair; beating or even leveling with nVidia is hard. But if anyone can show AMD and Intel how to do it....