Nope, AMD will own a big part of the gaming market, namely XB1, PS4 and the share of PCs with GCN cards.
A game coded for mantle as far as i understood would not have to be ported at all to run on a PC with a GCN GPU. The port to D3D then is only required for NV cards and older AMD cards.
Well, there's still the CPU and RAM configurations to account for, among other things.