To a certain extent it is a self fulfilling cycle.
Since it is currently the most popular device, more and more people will be purchasing it.
Since more and more people are purchasing them, there is more money to had in making apps for it than for comparable devices.
Since there are more apps being made for it than for other devices, more people will buy it.
I don't doubt that you're correct in that sooner or later something else will come along that is better, but at best that is probably at least a year or two away. When that happens the apps will follow the people.
I also share poohbear's concern in that the iPad and all its competitors may drive the market away from dedicated eReaders, which will raise their prices considerably and possibly impede future developments in them.