Are DPEs incentivized to fail people on checkrides so they can collect extra money on re-tests?
I think you are getting worked up about something that isn't a problem.  DPEs are incentivized to develop a professional reputation that upholds appropriate standards, while passing those who meet the standard.  Someone who is motivated by greed to fail students as you suggest would quickly get a bad reputation and people would simply stop scheduling check rides with him/her, so the plan would rapidly backfire.

Think about it, I could easily make the opposite case that I am concerned about safety because a DPE motivated by the desire to get a reputation of passing people the first time to increase his/her business could be tempted to compromise on the standards.  It should all balance out in the end.