PoW coins with low/no block reward have been shown to not work. A recent failure is Coin2, which was under massive attack and had to implement PoS after that. The attacker even posted in the ANN thread and claimed that he earned 20+BTC by 51% attacks. Considering Coin2 only has a very small market cap, 20BTC is a huge number. Quark could be the next Coin2 if we blindly believe Quark is unique and shouldn't be changed in anyways, and don't take any measure to increase network security.