The nature of the tablet is secrecy. If the Elder son wanted his ancestors to obtain power and overthrow the Senju he would have made it easier to decipher. Isn't it strange that the very ones who are following it's instructions stop before reading the final message? Isn't it interesting that the only ones capable of deciphering it's secrets have been wiped out (Uchiha Massacre).How about the Elder son?
He was hell bent on revenge after he lost the successor-ship to his younger brother brother and fought his younger brother, it would make sense if he wrote the tablet, in fact it would make more sense than the Sage himself. I do not believe the Sage would do up a tablet, it would be like a cope out to the younger son he chose as his successor.