I agree,it will make life difficult for devs who want to relrase their game on windows and android along eith ios.But ios users willd definitely see a huge improvement in performance and graphics over android users having phones of similar power or even slighly more raw power.The difference in gaming performance on a 5s and on an s5 will will significantly increase,unless devs intentionally gimp their games to make sure they run smoothly on android too.
Even among games not specifically iOS native, Metal will make a difference. It was designed in cooperation with the Unity and Unreal Engine teams, who will presumably optimize their engines to take advantage of it. That should make the performance gap between iOS and Android versions of the same apps even wider.
Some developers will gimp the iOS versions, but the ones who do will find themselves at a disadvantage against developers who instead maximize iOS performance.
All of this is exactly what Apple has in mind. They're still the number 1 target platform by a wide margin - I think they're trying to increase that distance even further.