Backwards compatibility is worth more then extra battery life. The A series CPUs are excellent and I am certain they cost Apple less then the Intel chips but adopting them for current products would cause too much grief. Intel is still improving their CPUs and not forcing Apple to switch architectures.
For new platforms, the A series CPUs are an obvious choice. I could see a version of MacOSX being ported to the latest large iPads. In time, the software ecosystem would develop to support the ARM CPUs and eventually, an ARM laptop could be viable. But this would take some time and would require Intel to drop the ball. I do not personally see it happening anytime soon.