Why was the naka shrine tablet written?

[Bijuu Sennin Uzumaki]

What if it wasn't written by the SOT6P, but instead by the eldest son.
Perhaps he couldn't beat the younger son when they fought, but the younger son didn't want to kill his brother.
Therefore, knowing full well that he couldn't beat his brother to acquire the full power of his father, he decided to make a tablet telling future generations how to go about it.
Perhaps, on the tablet, there is also a method for resurrecting the eldest son? Maybe he realized that his best chance to obtain the full power of his father would be to obtain the other half of the power from a weaker source?
This, perhaps, would also explain why stronger eyes are needed to read more.
The Elder brother wanted the later generations to desperately seek power. This way they would upgrade their eyes all the way to rinnengan. And only with this power could they fully (and not as an edo-tensei) resurrect him at his full power.

There doesn't seem much point in the SOT6P writing a tablet that would lead to bloodshed between his two sons (and their offspring's offspring's offspring...) detailing how to obtain his full power by using the other half's power...

 

[Your Creepy Stalker]

I think it was written as a message to Indra, or one of his reincarnations. Hagoromo knew indra would seek to become stronger so he would get better eyes and be able read to real message.

The final message is "When two opposing forces work together, real happiness can be achieved"
Indra was against cooperation, but Hagoromo and Ashura were for it. So it could have been written by one of them to make him see that cooperation is good to.