The recent studies that talk about this are slightly more nuanced. But much of it comes down to, what I suspect to be, some combination of the 60hr per week fallacy (that someone working 60hrs/wk creates more value than someone who limits their work to 40hrs/wk), the play trade off, the physical trade off, and the sleep trade off.
The brain needs lots of downtime to properly learn new skills, and if you're trading downtime for studying, at some point you're going to get negative returns on investment. This seems to happen somewhere between 40 and 50hrs/wk for cognitive tasks.
There's also a growing body of evidence that play, hard as it may be to define, really has an enormous impact on learning, as well as mental health. One of my favorite anecdotes is by Richard Feynman who was completely burned out when one day he, for the fun of it, tried to calculate the wobble of a plate being tossed in the air, and this changed his attitude with long lasting effects on his work. http://sistemas.fciencias.unam.mx/~compcuantica/RICHARD%20P.... (search for "throws a plate in the air", though the whole book is a fun read)
I'm convinced, though I've not seen much research on the topic, that physical movement is essential to learning. Something related to stimulating the entire nervous system, or possibly just related to increasing non-circulator fluid exchange in the brain. Hell, it may just be a chance for certain areas of the brain to have downtime, while the centers related to muscle control take their turn. But if you're making this trade off, it'll start to have negative returns.
And then there's sleep. You're doing yourself a great disservice if you switch an hour of sleep for an hour of studying, your long term retention will go to shit, and the odds of you remembering some fact 3 months from now will greatly diminish, which mostly defeats the purpose. This may be the biggest problem with giving homework to highschoolers, who tend to have the habit of both procrastination and staying up late.
So, yes, I can see homework having almost no academic benefit when a student is in class all day. I can already see issues with my daughter in Kindergarten. There's so much pressure to teach too much content, that her school has made huge tradeoffs around play and exercise during the school day. I certainly don't want them forcing even more work on her, especially as she's getting old enough to start to have chores and other responsibilities that have nothing to do with school.