You better choose some well defined set of propositions before you go off on this one. It's a very simple idea. People have been using non-boolean truth values since Bayes, and probably before. This is a matter of definition. And they still are, absolutely plausible.
Boolean logic works with sets of propositions which are definitely true or definitely false.
Surely you are not suggesting that people thought all language worked like that. That's both silly and historically false. More than useful. One can understand quantum mechanics perfectly, so long as one realises that the principle of superposition is part of a language for describing what may happen in measurement, not a description of a physical process. As such it is definitional truism, not something which need be derived from plausible argument or any other.