The early illusionist is a product of a different style of gaming, in which your class is not just a slight variant with one or two bonus powers, but a different way of approaching your adventuring. Your class determines the kinds of problems that you solve for the party, and how. +
In modern fantasy gaming, the illusionist rarely leans into this. Usually they're just wizards with a tendency to make illusionary rocks and monsters. Part of this is streamlining of rules, so the illusionist just works like other wizards. +
Part of it is a change in philosophy of play. Using illusionist spells effectively is hard. You have to figure out the motivations of your targets and then essentially run a con on them, in real time, on your feet, using only some trickery and fake images and sounds. +
If you can't play the illusionist at its difficult level, you don't have as much fun, so of course the class winds up bent until it's just a variant wizards that otherwise plays the same. If your DM isn't prepared to play that mode, you also feel useless when your spells don't work. +
The illusionist offers us an insight into one of the design problems that crops up in modern game design. Some old-school designs have a fair amount of friction. It takes some work to make them function. They require a concentrated level of play. +
Modern corporations want to minimize friction: They want the use of their product to be effortless, fast, easy; satisfaction guaranteed and instantaneous. So the illusionist winds up... slowly fading into obscurity until it's just a wizard with a different hat. ~Fin~