Although not a tablet in the current sense, I´ve been happily using my Maemo-powered ¨Internet Tablet¨ since before Apple made touchscreens.
The problem is tablet PCs, Nokia Internet Tablets and iPads are all different concepts of a 'tablet', confusingly called the same thing. Compare to Macs and PCs which are both clearly 'personal computers'.
Technically yes, but they were all considered failures. I recall that before it became clear what Apple was making (somewhere late 2009), there was a lot of speculation along the lines of, "well it can't possibly be a tablet PC, because nobody has ever had success with that".
Creating a market equals popularizing it.
A market of 10 people is not much of a market at all.
It wasn't a huge market, but it wasn't trivial either. The numbers I can find are around $1.5 billion/year sales in the mid-2000s.