Hey

I'll quote a statement from the Arena clarification thread.

"Yes, it is true! If you do not rank up, but you have already received the rewards for the Six Paths Sennin rank once, then you will not have to always be striving to the top. You might take a break from the Arena, or take it slow. Your rank will not reset, and you will keep winning the rewards for the rank in which you were!"

Practically, once you achieved Six Paths, regardless of the rank you get, you'll receive the proper reward for it.
And yes, we did reset the ranks. Players who reached Six Paths are now Rank Points 12