Also think of how long the shift from desktops to laptops took; for a decade they were just secondary machines and everyone who had a laptop also had a desktop. Now... the only person I know to even own a desktop built it himself 6 years ago and stopped upgrading it altogether a few years ago in favor of laptops.
A similar shift to tablets isn't as inevitable as the shift to laptops was (in hindsight), but if it's happening it's proceeding in much the same way, only faster.