Oh you mean the poneglyphs he read, but being the dumb pirate he is and not an oharan archeologist he was unable to understand them and came to whatever conclusion they did.
Dumb pirate or not, Roger used the language - Skypeia? - he had to have some way of deciphering them. I think you're undermining Roger a bit here, but that's just my opinion. He was the KING for a reason. He was capable of many feats, otherwise he wouldn't have conquered the Grand Line.
Oh and what did they do with their conclusion? They retired from pirating and did NOTHING. They didn't tell anyone else about what they discovered, they didn't fight any war to do whatever.
You're completely right, and I believe this is where Luffy surpasses Roger in his whole theme. Luffy is all about being free and helping people, unlike Roger who was hellbent on conquering the sea. Either way, this denotes a personality difference and does not in one way or another make it more or less plausible that Roger was able to read/comprehend a voice from the poneglyphs. Has it ever occurred to you that Roger's illness may have been a cause for the lack of action? Or maybe there were extenuating circumstances that prevented them from acting? Either way, we don't know the whole story. But we do know Roger could comprehend poneglyphs. Somehow. And the closest connection we have is Rayleigh's personal statement that they could hear the "voice of all things." This seems to be what Luffy has "inherited" so to speak.
Regardless of how much this whole inheritance thing seems to suck for y'all, ultimately Luffy's actions have drawn a direct contrast to those of Roger's. Just because he's following the same path, it doesn't mean he's doing the EXACT same things. It seems to me, he's inheriting the same things as Roger, and for better or worse, helps people and lives freely along the way instead of conquering the sea like Roger did. He's almost reversing the damage Roger may or may not have done to the world with this whole Era of Pirating thing. And he's doing it along the same path as Roger, showing not only a parallel, but also a better method of reaching the same goal - The End of the Grand Line.