"What if the bots apart from voting also make simple comments and receive massive upvotes from other bots?"

This wouldn't make economic sense. The bots would be better off self-voting or vote-trading this vote power more secretly. If they engaged in this to put themselves at the top of Steemian's blog posts, I think the community would quickly regulate and flag them.

Not much fear of being retaliated upon for flagging a bot!