The Road Poneglyphs

[SSJ2]

I'm kind of at a loss here with Oda's direction with the Road Poneglyphs. Initially I thought it was implied that Roger was able to understand the Road Poneglyphs due to hearing the voice of all things, but that has been proven wrong. The Kozuki clan are the only ones capable of reading the Poneglyphs (+ Robin), so how could any of the other Yonko hope to read them? Kaido was said to be interested in the Poneglyphs and wants to become Pirate King, yet he killed the only person capable of reading them, and now wants to kill his son. I don't get it.