When each new era arrives, the next group of users dwarfs the previous group. That happened when mainframes gave way to minis, and when minis gave way to PCs. The next era is always more approachable to a greater number of users who were intimidated by the previous era's tech.
So when I predict that by 2020 the majority (at least 51%) of the consumer (who knows what the business world will do by then, but I think many tablets will go there too) computer (not phone) purchase will be tablets. Not iPads, tablets. Not 2011 tablets, 2020 versions of tablets.
And many of those users in 2020 will be people who never did get a PC because they were intimidated by them, but they will love their touch tablets. The group of users expands.
As for how many of us will also have laptops and Macs/PCs in our homes, quite a few will. The Post-PC era does not mean PCs go away, any more than mainframes disappeared. Laptops will continue to be used where it makes sense to use them. But more tablets will be sold to consumers.