PowerPC wasn't all that efficient when they got knocked out of the consumer space. And once you're out, it's pretty hard to get back in, even if your IBM.
ARM took up their place as "efficient" CPU maker, and, well, we can see how that has gone for them. They won the mobile market, and are about to take over the rest of the consumer market -- at least if nVidia doesn't screw it up.
Intel was on the right track with Core, all the way until they got stuck at 14nm. Maybe this gets them back on track, or maybe our current manufacturing processes and technology mean we are hitting a dead end, I don't know.