I will answer very quickly: people who buy a tablet already have a PC. Stop dot.
Those devices are complementary but, in the fields where they are interchangeable, tablets allow to do the same while moving.
That's exactly why the netbook market has died: people who bought a netbook, already had a PC. And that's why the notebooks are struggling too, reinventing themselves as lighter, more powerful and cool ultrabooks: people who bought a notebook because they already had a PC, moved to tablets, but people who still need a laptop because they can't do without, now buy a cheap $499 one or a more glamorous ultrabook.
PCs will never really die: there will always be the need for them, after all.
