My company implements an optional every other Friday off. Somewhat surprisingly, Thursday hasn’t really become the new Friday. In fact, the people who choose to work the full 5 days tend to get more work done on Fridays, since fewer people are around to interrupt them. The people who take those Fridays off are generally happier and still pretty productive on Thursdays.

If people didn't want to work at all this might be true (at least if Friday off was widely implemented). But generally that doesn't seem to be the case. Unemployment seems to stress people, and most people seem to prefer vacations that are 2-3 weeks at a time, even if they could take more. So if you offered people to come in whenever they want they might be there more often than you expect.

Wouldn’t people be comparing to their peers outside of work? Most people don’t get any Friday’s off. It would be pretty wild to start wanting Thursday off so much when no one else is getting Friday off.

I’m not sure lazy people care which day of the week it is.