Tablets are great, but as long as they still have LCD (backlit etc.) screens they can't even be considered an alternative for anyone who actually does any serious reading. You can also pick up an ebook reader where I'm from for about 1/5th the price of even the most basic tablet.
If anything I think ebook readers are going to become even more popular. As e-ink or similar technology is refined and becomes even cheaper to produce I can easily imagine them becoming almost like disposable "throw-away" devices.
Also, in my opinion, your initial logic is incorrect. You talk as if "full-featured web-browsers like on the PC" is the "end-goal" for future reading devices. Well, we have always had that -on PC's- and they were the problem, not the solution. The e-ink display is the "solution" to the eye strain (among other things) that come with reading on a backlit display. So to suggest that is the goal seems to be as if we are going backwards.