Coase did his most relevant work in the 1950s, and it wasn't as if he invented the idea of externalities. It was first given serious academic weight in the 1890s, and Pigou created the concept of externality correcting taxes in the 1920s. His work was important because it proposed a more market based solution than Pigouvian taxes.

I think its safe to say that externalities are not, and were not, an ignored sector given more than a century of serious work, and the fact that it is covered in any intro level Econ course.

There's a wide gap between "ignored" and "received the attention they deserved".