Those who think mantle is going to eliminate dx need to look at the big picture. Many cpus, in fact the vast majority, are never used for demanding gaming. In fact I think Intel igps (without a dgpu) must be at least 60% of the market. Then you have nVidia which has at least 50 percent of the dgpu market, and actually the vast majority of the mobile dgpu market where the growth is. In any case I will be charitable and assume amd has 50% of the dgpu market. So thus you are down to a generous estimate that only 20 percent of the market has mantle capable hardware. This is not even considering that a portion of that 20% are not gcn (including all apus sold to date) and will not support mantle. In addition, not all new games will support mantle, and there are a huge number of legacy games that will not support it either.
So bottom line, there is a huge numbers barrier against mantle becoming the default graphics API, because it is hardware specific. As far as Intel and nVidia adopting it, I seriously doubt that will ever happen, both from a political standpoint and no one yet knows how much improvement it will bring to gcn, much less how it would work on Intel or nVidia hardware.