OP's point is that a POW/POS system with very small rewards creates a weak POW system that someone could exploit and it would essentially be the same as a standalone POS system which may be vulnerable.
It would not be the same as a standalone PoS system; rather, what it is is a PoS system plus a pathetically weak work PoW system which completely sidesteps aka does an end run around the PoS, so that the PoW attacker can attack without the PoS system getting in the way of the attack. Basically the PoS part is almost irrelevant given that the PoW attacker can do their attack and run with the loot before PoS even notices or acts?
Possibly the PoS might even lock into place the success of the attack, by building on a chain that already has the attack in place as having happened, over and done with, fait accompli ?
-MarkM-