What if you offer a 6-hour workday, and people who want to work less than 6 hours will apply... because this is still the best existing alternative for them? The retention goes up, but your employees will be unhappy, because they actually wanted less than 6 hours.

Meanwhile, the company that requires 8 hours will get a lot of people who are okay with working 8 hours (if that includes 2 or 4 hours on social networks), and a few people who are not okay with that, but still happier than your employees on average.