Sorry, JIM. That's legally not true. First off, registration is not the some total of trademark rights. Trademark rights arise from use in commerce. Obviously your statement in demonstrably false as two companies already have registered trademarks. They coexist because there's no likelihood of confusion between a pair of overalls and an industrial truck.

First off, the EAA never called it the "Oshkosh Air Show" or whatever. Hence they never really had trademark rights to that name. Further, a geographical mark has a weakness. You don't own the word "Oshkosh" by itself. Hence the EAA couldn't prohibit someone outside the gates selling T-shirts with the Word "OSHKOSH" and a picture of an airplane. They enforced it on the grounds by just making it a condition of your booth rental.

Airventure being a "fanciful" name is a much stronger mark. The problem, is that it really never achieved the level of fame that really allowed them to capitalize on it, because people continue to refer to it as Oshkosh (or the EAA depending on where you're from).