It's not as big of an issue as it might seem; most game engines have a well-designed layer of API abstraction anyway, so it's just a matter of implementing a new API backend and making sure it works properly. It's likely not a thing for small companies who write their own game engine and can't afford the QA necessary to add a new backend just for the sake of it.
Also note that Mantle is (supposed to be) HLSL compatible, which saves a lot of porting effort anyway.
Also note that Mantle is (supposed to be) HLSL compatible, which saves a lot of porting effort anyway.