They're gambling that ARM CPUs (SoCs) will become powerful enough to accomplish the tasks people ask of from Macs, while revenue from phone, tablet, and other small device sales (e.g. Apple TV) will be enough to sustain R&D to keep it progressing as rapidly as Intel CPUs.
It won't happen, and mainly for the exact reasons you stated. Phones and tablets have already taken over the "I don't do much other than browse the internet/watch youtube/update facebook/snapchat/twitter/email" jobs that low performance CPUs can handle. The only reason someone has a need to purchase a real computer now is because they have a real need for processing power (gaming, photo/video editing, developing software, running simulations). Everything else is already being done by the lightweight CPUs.