> As an counter example, consider how the decipherment of the mayan written language was performed
That is no counterexample. The Mayan written language was related to a number of surviving languages, making decipherment easier.
> Another one is the hieroglyphs.
The Egyptian hieroglyphs could not be deciphered until the discovery of the Rosetta stone, where the hieroglyphic text was accompanied by translations into Demotic Egyptian and Greek. Before that, there had been attempts for many centuries to decipher the hieroglyphs but they proved fruitless.
That is no counterexample. The Mayan written language was related to a number of surviving languages, making decipherment easier.
> Another one is the hieroglyphs.
The Egyptian hieroglyphs could not be deciphered until the discovery of the Rosetta stone, where the hieroglyphic text was accompanied by translations into Demotic Egyptian and Greek. Before that, there had been attempts for many centuries to decipher the hieroglyphs but they proved fruitless.