The right time would be at the height of Microsoft force feeding Windows 10 on non-suspecting Windows 7/8 customers... Sadly, there really is no worthy opponent.
OS-X - Nope, it would mean jumping into the fire, proprietary hardware linked to software, store regime, low security (sorry iSheeps, it is correct) and more.
Linux - Too many forks (in the road) for that to happen. Linux is actually Linux' worst enemy. If they start to unify and work together on one to three different blends it would go faster, but Linux seems to have to go through natural evolution and currently there are more versions than ever. In other words, it may never become a real contender, sadly.
Linux has been my hope for years, ever since Windows XP when it became clear Microsoft had an effective monopoly. Linux is in server rooms all over the world, but for client operating systems, it is an esoteric format still.