He's considered important in at least two fairly central, mainstream R&D areas. One, object-oriented programming, where his development of message-passing OO an

He's considered important in at least two fairly central, mainstream R&D areas. One, object-oriented programming, where his development of message-passing OO and the design of Smalltalk are an influential line of thought. And two, interactive, iterative development environments, which you can think of as attempts to create a more visual, structured next-gen of the REPL paradigm. It's true that his specific outputs there (Smalltalk, Squeak, etc.) have never become that mainstream. But: 1) they have become much more widely used than a lot of other research systems (Smalltalk is not just some PhD thesis used by 3 people), and 2) being even "highly influential" on two major areas of development that are mainstream-important is a pretty high bar to clear already.