Every RISC cpu is great for a particular level of complexity. And ARM is great for in order, no speculation, no branch prediction, ultra power sipping designs.
Through smart use of their instruction set they have made themselves great for ultra low power, low IPC, low frequency platforms. ARM for a PC misses the point of why x86 has been successful, its easy to tack crap on the side without resolving to use of "co-processors", and the less commonly thought about advantage of x86 the fact that x86 instruction code is basically a hardware interpreted byte code at this point. No modern x86 actually runs x86 is breaks it down or decodes it into a RISC setup that is suited to the particular level of complexity of the current generation/transister count/die shrunkenness/frequency.
If anyone truly cared to replace x86 they would design a fixed width CISC instruction set chip where the instructions are purposely designed to be a little higher level then risc and layed out in a way thats easy to decode into whatever the chip is actually doing the work in, so that when the next generation of chips come out you don't have to throw the whole thing out and start over like RISC has continually done over the ages.