1) ARM has never been efficient, just low power. ARM has gained in popularity because it is cheap technology to lease and manufacture, and the minimum power requirements have been low enough to get OK performance from portable devices. But minimum power usage just means low power usage, not efficiency. Efficiency is how much a processor can do with a given amount of power, and in that x86 reigns supreme and alone. With x86 processors coming out next year that will have comperable minimum power requirements I think we will see a surge of x86 phones and tablets flooding the market in 2015 and ARM will start to die down a bit.
2) The only company in the ARM community that can compete with Intel on R&D is Apple, and they will not lease their secret sauce to anyone, because they are apple and they never do. So in the future we will see Apple phones, tablets, desktops, and servers with ARM chips in them, and maybe a few other players like Samsung holding on to ARM simply because of the sheer amount of investment they have put in the platform over the years, but then just about everyone else will be running AMD or Intel x86 processors in everything.
Thankfully I think that Apple will be big enough to keep Intel in check with threats that they could start selling their chips to other device manufacturers, and Intel will always make sure that AMD does not go away so that Intel is not viewed as a monopoly. But I don't think that ARM in general will retain it's mobile dominance unless they somehow can compete with what Intel has coming down their pipeline.