Its just my Speculation, but I believe that the So6p saw the hatred and potential animosity between Senju/Uchicha etc. and as a last wish, he decided to troll them all.

I believe that the Rikudo specifically made the tablet FULLY readable to rinnegan possesors because he wanted more or less intermarriage between the Senju and the Uchicha. A half uchicha half senju would be fully capable of obtaining rinnegan extremely fast. This is all considering the pre residing facts that Senju Dna already enhances Uchicha techniques. I belive that the rikudo intended for atleast some hybrids of senju/uchicha. These persons powers would truly demonstrate the power of So6p.