A lot of server software is actually open source and runs on most commonly available hardware architectures, including ARM and PPC. You are right that everybody develops assuming x86, though, and there is some friction in that transition regardless of software portability.

In the sense that those processors can compute things which are computable, yes, they are a real alternative. The correct question is, "How much worse would the situation have to be with Intel before the cost of these alternatives was less than the benefit of switching to one of them." The answer, I guess, is: "much worse than things are now." However, if you're running at billions in revenue per quarter, you can afford to spend a few million to limit your downside risk by keeping yourself apprised of what the costs would be of moving to another platform.

More important is, are PPC or ARM competitive at the amount of computation per watt, and amount of computation per dollar. Power 8 and 9 are pretty good at number-crunching but also pretty expensive, and I don't know about power efficiency. ARMs used to be pretty power-efficient, but likely cannot offer very high single-threaded performance (must still be great for IO-heavy applications that are hit hardest on Intel).