When you look at books that introduce OO, and languages that incorporate it, it's usually presented as is-a and extends and inheritance.
The fact that Alan Kay meant something different is irrelevant.
While you can build messaging and composition into most languages, they're usually considered application models in their own right, not core features. (Objective C is an exception, among a handful of others.)
So I think it's a valid question - why do mainstream languages still present this model as a core feature when other models are less rigid, more expressive, easier to work with, and better candidates for language fundamentals?
The fact that Alan Kay meant something different is irrelevant.
While you can build messaging and composition into most languages, they're usually considered application models in their own right, not core features. (Objective C is an exception, among a handful of others.)
So I think it's a valid question - why do mainstream languages still present this model as a core feature when other models are less rigid, more expressive, easier to work with, and better candidates for language fundamentals?