Also, Intel tried with Itanium. It seems x86 is both a cash cow and a burden. What has kept Intel on top in the middle class CPUs is a monopoly, it's not like competing architectures are inferior per se. The Wintel alliance has given them guaranteed software and OS support, plus massive volume. To compete you would need massive volume or you're going to price yourself out of the market, but again x86 is proprietary, which meant competing architectures couldn't get the volume needed to hit the price sweet spot.
With mobile, the scale is shifting, you can now do 14nm with third party fabs (Samsung, Global Foundries). Add to this a plateuing of Moore's law, it may soon be a competitive performance edge with special purpose designs. It used to be that it was not worth it, because competitors could just wait 2 years and do nothing and a generic x86 would beat a special purpose design. It's not only ARM either, there's also new and novel ISAs like Risc-V approaching.