Actually, my Sybil attack can go something like this:
1) Vote for 1.5 MB blocks
2) Check how many nodes I have killed with the increase
3) Vote for 2MB blocks
4) Check how many nodes I have killed with the increase
X) Keep doing until I am the network of nodes. Fill up the blocks and start pushing out the Chinese miners as well.
Incremental Sybil attack, cheap and easy to pull off.
You can do the same with Core by modding it. Unless you're making assumptions about how people would configure their settings. Sure, if you think people will do unprofitable things without Core supervision, then yeah that could happen. I tend to think people with skin in the game will not do anything silly. A few may try, lose money, end of story - in fact this happened at the halving in November 2012 when a few miners modded their Core clients to keep getting 50 BTC per block. They lost $500 per pop, then "we never did that again."