I'm totally fine with the position that an employer can't reliably get 8 hours of productive work per day from employees. I just am a bit fed up with people repeating over and over again variations on the theme of "nobody doing creative work can consistently be productive for 40 hours per week" which is something I have personally observed multiple counterexamples from across several industries.
...and it is OK. Yes, you have to pay someone for 8 hrs in order to get 4 hrs of useful work for you company out of them! Yes you have to pay for their side projects even if you like to pretend you don't! And yes you have to pay for people slacking at work... otherwise you don't even get that 50%!
So, as an employer, accept this fact, pay up, and motivate your people to create awesome products that delight the consumers and bring record profits, in this limited amount of useful time that you can squeeze out of people!