Yes everyone would do well to keep in mind that the first time that anything was called “episode” was with the release of Empire Strikes Back in 1980, where it was abruptly called Episode V with no explanation. Lucas claims to have planned out 6 movies, 9 movies, 12 movies, or whatever, but the other main purpose of it was to hearken back to the experience of going to see serials like Flash Gordon in the theater and starting in the middle with whichever one you happened to catch - if you missed a previous episode, it was just gone. That was also the purpose of having title crawls, to catch you up on any exposition that you probably missed.

The idea of having something missing was intentional. Strictly speaking, the prequels are just as unnecessary to the structure as anything else. They were themselves a fun thought experiment, there was no reason to expect that they might get made.