My iPad couldn't replace my laptop or my desktop systems. It can and did replace the small 11" netbook+ that I used when I was squeezed into a middle seat in coach for the last couple of years. And while it hasn't replaced my smartphone, the "smarts" of the phone go largely unused since I purchased my iPad.
Happened to read an article a few weeks ago when the Amazon Fire was introduced that captured my attitude. The author noted that the Fire promised to fill his personal technology "gap" well. By that he meant that his lightweight but powerful laptop covered many of his needs for content creation for work and pleasure. His smartphone was his "go to" portable device for communication and quick access to the internet. His personal "gap" consisted of functionality the Fire promised to fill very well with relatively little redundancy.
My own situation is a bit different but the author's point was valid. There is no "universal" device equally capable of meeting everyone's requirements. The trick is to identify one's own "gap" and fill it with as little redundancy (i.e. expense) as possible. The requirements laid out by the OP suggest that his/her "gap" is more than adequately filled with an iPad. That's true of millions of consumers who were forced to "overbuy" technology until the iPad came along.
On the other hand, I think those who experience disappointment with the iPad don't adequately consider their needs in the first place. It's easy to be seduced by the shiny iPad and sometimes easy to forget the features (sometimes subtle) that one depends upon with other devices. (I cannot get along without multiple resizable windows and dual monitors for much of my work but I have to admit that if I were to list my computing requirements, I might not put that at the top of my list until I thought about how I do my work.)
For me, at least, the iPad is a largely redundant device that I could have have foregone. What it does well can be done nearly as well or better by other systems I already had. But it does have one extraordinary feature that no other device can match. It enables me to share valuable time with my seven year old daughter as we explore the world together. And that makes it priceless despite its redundancy. In other words, that was the true "gap" it filled for me.