if you lower it too much, you get people just voting for themselves

Yes that's true, people would probably self-vote. But besides it being frowned upon, their VP would diminish quickly and their rewards would be almost worthless. It would eventually not be worth the effort. They also would drain their resource credits, which would put a stop to any excessive drain for the servers. To get around it, they would have to buy HIVE and power up, and that's not a terrible thing.