Either that or results-based work doesn't actually help us get things done faster than traditional work (_i.e._ it's snake oil) and at best it'll limp along providing little benefit to anyone but the people selling books about it.
So either it will fail/stagnate because it doesn't improve productivity, or it will become the new norm and we'll just have enough results expected of us that the effect on workers is at best indistinguishable from traditional work (businesses that fail to fill up the free time with more work will, generally, lose to those that do)
That hasnt been my experience. My employer doesnt care how or when throughout the week I get something done. Just once a week I state what I accomplished and what I am working on now. We still have deadlines, just no constant helicoptering. Its obvious when a project seems behind and I need to allocate extra hours to accomplish it.
It does result in plenty of flexibility and free time.
Constant checkin's, helicoptering, and meetings are nothing but an impediment to productivity IMO. If your company needs that as part of their workflow they prob need to reassess their hiring process (not hiring A players) and communication processes.
9-5 isn't the 'optimal'. It's simply what is 'most common' for a diurnal society. Nobody is driving around town looking to get a hair cut at 3am and staring at an IDE from 1pm-5pm trying to solve a bug isn't making you any more efficient than thinking about the problem while cooking an early dinner.
Humans do suffer fatigue - both mental and physical - and proper lengths of rest helps both kinds.
I posit that happier employees are more likely to trend their work/life balance in favor of work if they enjoy their work and enjoy their company. Unhappy employees will skirt by on the bare minimum required knowing that hiring/training a new employee isn't worth the cost of firing them.