Are the people advocating working for less time also ok with getting paid less as well?
I think that is what it always comes down to and I am pretty sure the answer is a hard no.
I think people want to work for less time while being paid the same.
The reasoning behind this is because for office work at least, a lot of people are only productive for about 5-6 hours (if that) out of those 8 hours, and space out or do other tasks to recover/relax for the rest, because it's very difficult to crank out work continuously for 8+ hours straight day after day. So from the perspective of the employer, you're still getting around the same 6 hours of useful work out of your employees regardless of whether they're in the office for 6 hours or 8 hours.
The argument for having your workers come in and stay for 6 hours then makes more sense for both the employee and the employer. The employer gets the same amount of work from their employee, except the employee is more rested and happier, which reduces the rate of human error, sick days, and turnover, while improving the quality of work. All of which measurably save money for the employer, while spending the same amount on salary.
For the employee, the obvious improvement in quality of life means they are happier, more productive, don't resent their work as much, if at all, and are healthier. It also reduces possible vices that people may turn to as a way to soften the blow from shitty working conditions, such as alcohol, eating, drugs, and so on.