I highly doubt this will happen. Even if peercoin has the same marketcap as bitcoin, i doubt we will see this occur. Only if peercoin’s marketcap is much bigger than bitcoin will we see the turning point.
Mining is for profit. We will see the turning point when the rewards are equal. So the condition is:
(Btc fees + btc reward) * btc price = (ppc PoW reward) * ppc price
As ppc reward decreases with hash power while btc reward only decreases discontinuously over a long period of time, and btc fees only seem to go up with time, it would require a very high ppc price to see this turning point occur.
This is actually a good thing, because this turning point also implies that peercoin’s energy consumption would be overtaking bitcoin’s. That will not ovcur when their marketcaps are equal, but instead only when peercoin has a much larger marketcap than bitcoin.